1 00:00:01,668 --> 00:00:04,770 [narrator] Scientists believ there is a hidden substance deep in space 2 00:00:04,772 --> 00:00:07,106 that keeps the cosmos runnin 3 00:00:08,175 --> 00:00:11,043 But is that substance real? 4 00:00:11,045 --> 00:00:14,380 We've never seen dark matter, it's completely invisible, 5 00:00:14,382 --> 00:00:17,349 but we know that it has to be there. 6 00:00:17,351 --> 00:00:22,454 Not only can you not see it, you couldn't really touch it or taste it, or smell it, 7 00:00:22,523 --> 00:00:25,657 and yet it is all around us it affects everything that we do. 8 00:00:25,726 --> 00:00:27,659 [narrator] After searching for decades, 9 00:00:27,728 --> 00:00:32,131 we still don't understand this inexplicable substance 10 00:00:32,133 --> 00:00:36,835 [de Rham] We know dark matte is there because we feel its strong gravitational pul 11 00:00:36,837 --> 00:00:39,671 but it just doesn't want to talk to us. 12 00:00:39,673 --> 00:00:41,573 [narrator] There's evidence that dark matter 13 00:00:41,575 --> 00:00:45,577 makes up 85% of all the matter in the universe. 14 00:00:45,646 --> 00:00:49,148 We can see dark matter holding galaxies together 15 00:00:49,183 --> 00:00:53,986 and ripping other structures apart, we even see it bending light. 16 00:00:53,988 --> 00:00:58,624 Dark matter itself has been around since the beginning of the universe. 17 00:00:58,626 --> 00:01:02,528 Without dark matter, we wouldn't be here. 18 00:01:02,530 --> 00:01:04,696 [narrator] But if you can't see dark matter 19 00:01:04,698 --> 00:01:09,001 and if you can't touch it, does it really exist? 20 00:01:22,917 --> 00:01:25,484 [narrator] The Hyades star cluster. 21 00:01:26,854 --> 00:01:32,858 This family of 700 stars is 150 light years from Eart 22 00:01:32,860 --> 00:01:36,261 At the scale of the universe it's in our backyard. 23 00:01:36,997 --> 00:01:38,831 Hyades is actually close enough to Earth 24 00:01:38,833 --> 00:01:39,965 that you could see it with your naked eye. 25 00:01:39,967 --> 00:01:42,201 When you look up at the night sky, 26 00:01:42,203 --> 00:01:45,938 Hyades is in that V-shape in Taurus the Bull. 27 00:01:47,608 --> 00:01:50,943 [narrator] For most of its 650-million-year lifetime, 28 00:01:51,011 --> 00:01:54,680 the Hyades enjoyed a peaceful existence. 29 00:01:54,682 --> 00:01:58,050 But something is breaking the calm. 30 00:01:58,118 --> 00:02:01,620 The Hyades cluster is one of the most well-studied clusters of stars 31 00:02:01,622 --> 00:02:03,655 we have in the entire sky and yet there's something 32 00:02:03,724 --> 00:02:06,325 very deeply mysterious going on. 33 00:02:07,361 --> 00:02:11,063 [narrator] Two star tails extend from the cluster center, 34 00:02:11,098 --> 00:02:17,669 they should be roughly equal but one tail is hemorrhaging stars. 35 00:02:17,671 --> 00:02:20,672 Something is disrupting it, there's something exerting a force on it 36 00:02:20,674 --> 00:02:24,109 that's ripping stars out of their orbits. 37 00:02:25,880 --> 00:02:28,747 [narrator] Something with immense gravitational pull 38 00:02:28,815 --> 00:02:30,449 has passed by the cluster 39 00:02:31,252 --> 00:02:33,585 and robbed it of stars. 40 00:02:33,587 --> 00:02:37,890 In order to be gravitationally pulling stars out of an object like Hyades, 41 00:02:37,892 --> 00:02:39,725 you need to have an incredibly massive structure, 42 00:02:39,760 --> 00:02:42,995 as much as 10 million times the mass of the sun. 43 00:02:44,031 --> 00:02:48,000 [narrator] This monstrous cosmic mugger should still be visible, 44 00:02:48,502 --> 00:02:50,602 but when we point our telescopes 45 00:02:50,604 --> 00:02:55,073 to where it should be, that region is empty. 46 00:02:56,110 --> 00:03:00,212 There's nothing there and I mean nothing. 47 00:03:00,214 --> 00:03:04,082 And not a little bit or something dark, or something small, 48 00:03:04,084 --> 00:03:06,218 but there's literally nothing that we can see. 49 00:03:07,788 --> 00:03:12,758 [narrator] We know somethin is out there, invisible and powerful. 50 00:03:12,760 --> 00:03:15,894 And whenever we witness these unseen assaults, 51 00:03:15,930 --> 00:03:18,830 a prime suspect gets called in, 52 00:03:18,832 --> 00:03:20,299 a phantom of physics, 53 00:03:21,569 --> 00:03:23,302 dark matter. 54 00:03:24,772 --> 00:03:31,410 So what can we confidently s about this mysterious cosmic substance? 55 00:03:31,412 --> 00:03:37,082 It does not emit light, it does not reflect light, it does not absorb light. 56 00:03:37,084 --> 00:03:40,886 The only thing we know about dark matter is that it has gravity. 57 00:03:40,888 --> 00:03:42,521 We're not even really sure it's matter at all. 58 00:03:42,523 --> 00:03:45,290 It's just that that's the only thing we know, that it has gravity. 59 00:03:45,359 --> 00:03:49,361 [narrator] We may not be abl to see or touch dark matter 60 00:03:49,363 --> 00:03:54,666 but we are very good at finding its fingerprints all over the universe. 61 00:03:54,735 --> 00:04:00,038 We can see dark matter's use of gravity to break and bind structures 62 00:04:00,107 --> 00:04:03,475 and we've been spotting its handiwork for decades. 63 00:04:04,845 --> 00:04:08,013 Let's rewind back to 1933. 64 00:04:09,483 --> 00:04:13,952 Swiss-American physicist, Fritz Zwicky tracks strange movements 65 00:04:13,954 --> 00:04:18,457 in a far off collection of galaxies called the Coma cluster. 66 00:04:18,459 --> 00:04:22,928 He knows he's not seeing the whole picture. 67 00:04:22,930 --> 00:04:28,767 Some galaxies are speeding around the cluster at inexplicably fast rates. 68 00:04:28,769 --> 00:04:30,669 [Bullock] Zwicky is looking at these galaxies 69 00:04:30,671 --> 00:04:34,539 and if the only mass that was there were the other galaxies you can see, 70 00:04:34,608 --> 00:04:38,310 you would expect these galaxies to be moving at about 50 miles a second, 71 00:04:38,312 --> 00:04:41,613 then they would stay bound to each other and not fly apart. 72 00:04:41,615 --> 00:04:45,284 Instead, he sees them moving at 1,000 miles a second. 73 00:04:46,720 --> 00:04:48,487 [narrator] At these velocities, 74 00:04:48,489 --> 00:04:53,225 galaxies should be flying of the cluster like sparks from fireworks. 75 00:04:54,928 --> 00:04:58,997 Zwicky realized there had to be extra stuff, 76 00:04:59,333 --> 00:05:03,101 in his words, Dunkle Materie 77 00:05:03,136 --> 00:05:04,903 Dark matter. 78 00:05:04,905 --> 00:05:05,303 Dark matter. 79 00:05:05,305 --> 00:05:07,372 Dark matter. 80 00:05:07,374 --> 00:05:13,812 [narrator] It becomes clear that Zwicky's Coma cluster isn't an isolated case. 81 00:05:13,880 --> 00:05:19,918 Astronomers begin seeing the same dynamics within galaxies themselves. 82 00:05:19,986 --> 00:05:23,121 In systems governed exclusively by gravity, 83 00:05:23,190 --> 00:05:25,290 objects farthest away from the center 84 00:05:25,359 --> 00:05:27,959 would take the longest to complete an orbit. 85 00:05:27,961 --> 00:05:32,497 But in many galaxies, stars on the outside are orbiting 86 00:05:32,499 --> 00:05:36,301 at almost the same rate as those in the core. 87 00:05:36,303 --> 00:05:37,802 [Thaller] It's almost like a photograph record. 88 00:05:37,804 --> 00:05:41,873 Every part of that record spins around like a solid disc. 89 00:05:41,875 --> 00:05:46,411 The stars are going too fast to stay bound to the gravity of the galaxy. 90 00:05:46,413 --> 00:05:49,014 They should just fly right off into space. 91 00:05:49,516 --> 00:05:53,251 [narrator] Physicists come u with an explanation. 92 00:05:53,287 --> 00:05:59,758 Galaxies sit in a giant hal or ball of invisible dark matter. 93 00:05:59,760 --> 00:06:04,096 And it's that extra mass that allows the stars to turn fast 94 00:06:04,098 --> 00:06:06,932 all the way out to the galactic rim. 95 00:06:06,934 --> 00:06:08,633 [Thaller] Think about actual taking a disc of dough 96 00:06:08,635 --> 00:06:10,769 and spinning it to make a pizza. 97 00:06:10,837 --> 00:06:15,474 The more you spin it, the more those outer regions go farther and farther away. 98 00:06:15,476 --> 00:06:17,743 Eventually, the dough just goes flying everywhere, 99 00:06:17,811 --> 00:06:21,179 that's what would happen to a galaxy if it weren't for dark matter. 100 00:06:21,181 --> 00:06:24,049 Uh, as you spin pizza dough and you spin it faster and faster, 101 00:06:24,051 --> 00:06:25,884 it does hold itself together 102 00:06:25,886 --> 00:06:30,522 because there's all this yummy gluten that's acting as a glue. 103 00:06:30,590 --> 00:06:33,992 Dark matter is the gluten of our universe. 104 00:06:34,728 --> 00:06:37,729 [narrator] By calculating the mass needed to bind 105 00:06:37,731 --> 00:06:40,866 those speeding outer stars to the galaxy, 106 00:06:40,934 --> 00:06:43,001 physicists are able to estimate 107 00:06:43,003 --> 00:06:47,172 how much visible matter there is compared to dark matter. 108 00:06:47,174 --> 00:06:49,808 The results are staggering. 109 00:06:49,876 --> 00:06:53,879 [Tegmark] All the stuff we thought existed was just maybe 15% of our universe. 110 00:06:53,881 --> 00:06:56,014 That's like if you go to a restaurant 111 00:06:56,016 --> 00:07:00,619 and leave like the measly 15% tip, you know, that's what we are. 112 00:07:00,621 --> 00:07:02,421 I mean, not even the majority substance. 113 00:07:03,590 --> 00:07:05,490 [narrator] We may not be able to see it, 114 00:07:05,492 --> 00:07:10,061 but dark matter makes up some 85% of all matter. 115 00:07:10,096 --> 00:07:14,633 Wherever we look, we can se its gravity having effects. 116 00:07:14,635 --> 00:07:18,837 It glues galaxies like our Milky Way together 117 00:07:18,905 --> 00:07:25,076 And a close look reveals dark matter can also bend light itself. 118 00:07:26,747 --> 00:07:29,581 It's called gravitational lensing. 119 00:07:29,583 --> 00:07:32,717 A massive object can bend space and time, 120 00:07:32,719 --> 00:07:38,723 and light must follow the curves of that space and time. 121 00:07:38,725 --> 00:07:43,829 [narrator] Gigantic clumps of any matter create a gravitational lens. 122 00:07:43,897 --> 00:07:48,600 Dark matter showed its space-warping power in a trick it played 123 00:07:48,602 --> 00:07:53,305 with a gigantic explosion in a far off galaxy cluster 124 00:07:54,975 --> 00:08:00,011 Supernova Refsdal was first detected in November of 2014 125 00:08:00,981 --> 00:08:04,983 Supernova Refsdal is actually one of my favorite recent results 126 00:08:04,985 --> 00:08:07,419 in all of the astronomical literature. 127 00:08:07,421 --> 00:08:08,987 That result blew me away. 128 00:08:09,022 --> 00:08:12,858 So a star explodes, light is emitted in all directions, 129 00:08:12,860 --> 00:08:15,961 and some of it makes its way towards the Earth. 130 00:08:15,963 --> 00:08:18,697 So far so good. This is very standard. 131 00:08:18,765 --> 00:08:20,232 So the flash appears 132 00:08:20,968 --> 00:08:23,335 and then, another flash appears. 133 00:08:24,471 --> 00:08:28,940 We see it again, and again, and again. 134 00:08:28,942 --> 00:08:32,878 [Mingarelli] We see the explosion go off in four parts of the sky. 135 00:08:32,880 --> 00:08:36,114 And then, a year later, a fifth explosion goes off 136 00:08:36,182 --> 00:08:38,083 in a totally different part of the sky. 137 00:08:38,085 --> 00:08:39,985 What's going on? 138 00:08:39,987 --> 00:08:45,657 [narrator] Analysis proves that these multiple explosio are the same supernova. 139 00:08:45,659 --> 00:08:50,128 -[explosion] -But between this one dying star and our telescope 140 00:08:50,130 --> 00:08:53,398 sits a giant mass of dark matter, 141 00:08:53,400 --> 00:08:56,835 a huge gravitational lens. 142 00:08:56,837 --> 00:09:01,506 What that means is that some of these rays of light will take much longer, 143 00:09:01,575 --> 00:09:05,277 more complicated paths through this region of space time. 144 00:09:05,946 --> 00:09:09,514 [narrator] The dark matter lens turns one supernova 145 00:09:09,583 --> 00:09:14,219 into a fireworks display lasting an entire year. 146 00:09:15,489 --> 00:09:19,758 Dark matter affected the trajectory of light from this supernova so much 147 00:09:19,826 --> 00:09:21,860 that for some of those trajectories, 148 00:09:21,895 --> 00:09:24,462 it added a whole light year, 149 00:09:24,531 --> 00:09:28,433 it took a whole extra year for light to reach us. 150 00:09:29,603 --> 00:09:32,837 [narrator] Something is ver definitely out there 151 00:09:32,906 --> 00:09:35,440 distorting our view of the cosmos. 152 00:09:36,677 --> 00:09:41,112 It's a potent clue that dark matter is real. 153 00:09:41,114 --> 00:09:47,085 Now, new evidence suggests that without it, we might not exist at all. 154 00:09:54,861 --> 00:09:58,263 [narrator] The cosmos is filled with an unseen substance, 155 00:09:58,498 --> 00:10:02,734 its mass even bends starligh 156 00:10:02,769 --> 00:10:09,207 Gravitational lensing sugges dark matter holds our entir universe together. 157 00:10:10,510 --> 00:10:14,646 For decades, this specter of space has haunted us. 158 00:10:14,714 --> 00:10:17,782 We've never been able to pin it down. 159 00:10:17,784 --> 00:10:23,922 In 2021, an international te ran a virtual experiment to try to predict 160 00:10:23,924 --> 00:10:27,626 where dark matter should be by letting computers 161 00:10:27,694 --> 00:10:30,395 map out where we think it lives. 162 00:10:30,463 --> 00:10:32,998 [Bullock] Because we think we know how it behaves, 163 00:10:33,066 --> 00:10:36,501 we can model what it should be doing in supercomputer simulations. 164 00:10:37,671 --> 00:10:42,941 [narrator] The team taught the computer how dark matte bends light, 165 00:10:42,943 --> 00:10:49,981 then applied computational power to 17,000 unexplored galaxies. 166 00:10:50,651 --> 00:10:55,553 The model created a dark matter map. 167 00:10:55,555 --> 00:10:58,723 I think a lot of people, when they imagine the universe on the larger scales, 168 00:10:58,725 --> 00:11:01,026 think it's sort of boring, everything's uniform. 169 00:11:01,028 --> 00:11:03,495 But that's not what we see. 170 00:11:03,497 --> 00:11:06,431 What's amazing is that on the larger scales of the universe, 171 00:11:06,433 --> 00:11:09,100 we see a very particular pattern. 172 00:11:09,102 --> 00:11:12,470 When we zoom out, we see this magnificent structure, 173 00:11:12,472 --> 00:11:15,373 this cosmic web that's created by dark matter. 174 00:11:16,576 --> 00:11:19,277 [narrator] The interweaving tendrils of dark matter stretch 175 00:11:19,279 --> 00:11:23,882 for thousands of light year across the cosmos. 176 00:11:23,917 --> 00:11:30,088 At the junctions where matte is concentrated, we find galaxies form, 177 00:11:30,090 --> 00:11:32,457 illuminating the dark scaffold. 178 00:11:34,061 --> 00:11:36,728 If dark matter exists, scientists believe 179 00:11:36,730 --> 00:11:40,932 it makes up 85% of the matte in the universe, 180 00:11:41,000 --> 00:11:45,570 and also controls the remaining 15% regular matter, 181 00:11:45,638 --> 00:11:49,174 like stars, planets, us. 182 00:11:50,477 --> 00:11:54,245 If they're right, dark matte played a critical role 183 00:11:54,247 --> 00:11:58,383 in actually building the universe we see today. 184 00:12:03,056 --> 00:12:04,489 2021. 185 00:12:04,491 --> 00:12:08,226 Astronomers using the SkyMapper observatory in Australia 186 00:12:08,228 --> 00:12:14,733 trains specialist optics on a dwarf galaxy called Tucana II. 187 00:12:14,735 --> 00:12:20,472 The SkyMapper's filters split up the starlight into a spectrum of wavelengt 188 00:12:20,474 --> 00:12:24,342 revealing some very ancient light. 189 00:12:25,746 --> 00:12:30,181 [Tremblay] One of the best clocks that we can put on the universe 190 00:12:30,250 --> 00:12:32,283 is the progress of chemistry. 191 00:12:32,385 --> 00:12:33,451 Right? 192 00:12:33,453 --> 00:12:37,522 The build-up of more complex elements over time. 193 00:12:37,524 --> 00:12:41,893 Stars are nothing if not factories of chemical complexity. 194 00:12:41,895 --> 00:12:45,597 They slam, uh, particles together and create heavier elements, 195 00:12:45,665 --> 00:12:48,233 right, through a process called fusion. 196 00:12:48,268 --> 00:12:52,470 The later the generation of star, the more chemicall complex it is. 197 00:12:52,472 --> 00:12:56,608 [narrator] Tucana II's spectral signature reveals its stars contain 198 00:12:56,676 --> 00:13:00,512 very few of these heavy complex elements. 199 00:13:00,514 --> 00:13:05,817 A clue that lets astrophysicists calculate the age of the galaxy. 200 00:13:05,885 --> 00:13:10,688 These are very, very old stars from the very early days of the universe 201 00:13:10,690 --> 00:13:15,260 when the gas in the universe was not that chemically complex. 202 00:13:15,962 --> 00:13:19,931 Tucana II might be one of the oldest known structur 203 00:13:19,999 --> 00:13:21,900 that we can see in our local universe. 204 00:13:21,968 --> 00:13:25,336 It could be as old as 13 billion years. 205 00:13:25,338 --> 00:13:28,106 You know, almost as old as the universe itself. 206 00:13:29,843 --> 00:13:33,278 [narrator] This grand old la of a galaxy is a tiny thing 207 00:13:33,313 --> 00:13:35,380 Barely 3,000 stars. 208 00:13:37,083 --> 00:13:39,584 And yet, way out on her galactic rim, 209 00:13:39,652 --> 00:13:43,988 stars hurdle around at breakneck speed. 210 00:13:43,990 --> 00:13:47,492 [Plait] When you look at the mass of this ultra-faint dwarf galaxy, 211 00:13:47,494 --> 00:13:50,695 it only has a few thousand times the mass of the sun. 212 00:13:50,697 --> 00:13:52,263 That's really small. 213 00:13:52,265 --> 00:13:54,899 And at the speed it's moving, it should fly apart. 214 00:13:54,901 --> 00:14:00,004 [narrator] Tucana II doesn' break up because it's glued together, 215 00:14:00,006 --> 00:14:04,576 apparently by an incredible amount of dark matter. 216 00:14:04,578 --> 00:14:06,644 When you look at a galaxy like our Milky Way, 217 00:14:06,646 --> 00:14:10,048 it's about 85% dark matter, which is a lot. 218 00:14:10,050 --> 00:14:14,085 But with Tucana II, it's more like 99%. 219 00:14:14,087 --> 00:14:18,356 [narrator] Tucana II is old among the oldest galaxies in the universe 220 00:14:18,358 --> 00:14:22,360 and it is packed full of dark matter. 221 00:14:22,362 --> 00:14:27,432 Simulations suggest this dark matter played a ke role in shaping Tucana II 222 00:14:27,434 --> 00:14:31,569 and other very early galaxie right from the beginning, 223 00:14:31,571 --> 00:14:38,843 gathering regular matter into clumps and building the first galaxies. 224 00:14:38,845 --> 00:14:41,012 [Thaller] The importance of dark matter really can't be overstated. 225 00:14:41,014 --> 00:14:44,449 It has actually controlled the way matter has evolved 226 00:14:44,517 --> 00:14:45,483 since the beginning of the universe. 227 00:14:45,551 --> 00:14:47,352 It brings matter together. 228 00:14:47,354 --> 00:14:53,091 You need this underlying structure of dark matter to make it all happen. 229 00:14:53,093 --> 00:14:58,463 [narrator] Scientists think that for billions of years as the early universe grew, 230 00:14:58,465 --> 00:15:00,665 dark matter called the shots 231 00:15:00,667 --> 00:15:06,104 Without its gravity, structures like the Milky Wa wouldn't have formed. 232 00:15:07,807 --> 00:15:11,142 We've seen dark matter's light-bending effects. 233 00:15:11,144 --> 00:15:14,445 We've even deduced where it should be. 234 00:15:14,447 --> 00:15:18,449 Dark matter really does appear to exist, 235 00:15:18,451 --> 00:15:22,287 but this evidence is indirect, circumstantial 236 00:15:23,156 --> 00:15:27,492 To get conclusive proof that dark matter exists, 237 00:15:27,494 --> 00:15:29,227 don't we need to find some? 238 00:15:29,663 --> 00:15:33,898 If we could find a lump of dark matter [laughs], 239 00:15:33,967 --> 00:15:37,101 um, that would be one of the greatest discoveries in all of nature, 240 00:15:37,103 --> 00:15:38,536 in all of our history, right? 241 00:15:38,538 --> 00:15:40,037 Because we would understand 242 00:15:40,273 --> 00:15:44,275 one of the most fundamental components for how our universe works. 243 00:15:44,811 --> 00:15:47,011 Dropping the title, they love that. 244 00:15:48,148 --> 00:15:52,216 [narrator] It's time to hun for dark matter itself. 245 00:15:52,218 --> 00:15:56,454 Could it be hiding in the darkest place of all 246 00:15:57,791 --> 00:15:59,424 Black holes. 247 00:16:09,569 --> 00:16:11,869 [narrator] Scientists believ an invisible substance 248 00:16:11,938 --> 00:16:15,873 is pulling the strings in our universe. 249 00:16:15,942 --> 00:16:19,877 But until we see it, sense i perhaps even touch it, 250 00:16:19,946 --> 00:16:22,447 dark matter is just a theory 251 00:16:23,850 --> 00:16:26,017 Sometimes though, ideas dreamed up 252 00:16:26,019 --> 00:16:28,319 by scientists come true 253 00:16:28,822 --> 00:16:31,956 like black holes. 254 00:16:31,958 --> 00:16:35,960 Once the stuff of science fiction and children's nightmares, 255 00:16:35,962 --> 00:16:40,765 black holes today are confirmed reality. 256 00:16:40,767 --> 00:16:43,935 So black holes and dark matter have a ton of similarities, right? 257 00:16:43,937 --> 00:16:46,471 You know, an unseen collection of matter 258 00:16:46,473 --> 00:16:48,873 that creates an enormous gravitational field, check. 259 00:16:48,908 --> 00:16:52,877 It bends light and causes gravitational lensing, check. 260 00:16:52,912 --> 00:16:56,347 Tests the boundaries of known physics, check. 261 00:16:57,617 --> 00:16:59,951 [narrator] It seems crazy to even ask, 262 00:16:59,953 --> 00:17:01,853 but could our search for dark matter 263 00:17:01,855 --> 00:17:06,591 end in an idea more than 100 years old? 264 00:17:06,593 --> 00:17:10,094 Could dark matter be black holes? 265 00:17:11,664 --> 00:17:14,565 Black holes appear when stars explode. 266 00:17:14,567 --> 00:17:17,769 And their remaining mass crunches down into a sphere 267 00:17:17,837 --> 00:17:22,840 so dense even light can't escape its gravity. 268 00:17:22,842 --> 00:17:27,345 But that's where the black hole dark matter theory stumbles. 269 00:17:27,347 --> 00:17:29,614 We know that black holes happen. We know how they form. 270 00:17:29,682 --> 00:17:34,919 And we also know that there's nowhere near enough of them to be dark matter. 271 00:17:34,921 --> 00:17:38,723 [narrator] Not enough stars have lived and died in the history of the univer 272 00:17:38,725 --> 00:17:42,927 to create 85% of the matter in it. 273 00:17:42,962 --> 00:17:46,230 If dark matter is made up of black holes, 274 00:17:46,232 --> 00:17:50,668 they would have to be an entirely new type. 275 00:17:50,670 --> 00:17:55,673 It's possible that these black holes are of a type that we've never seen before. 276 00:17:55,741 --> 00:17:59,143 They could be primordial black holes. 277 00:18:00,580 --> 00:18:03,681 [Hopkins] Primordial black holes are an idea. 278 00:18:03,683 --> 00:18:05,483 A theoretical concept at this point 279 00:18:05,485 --> 00:18:10,154 that we've never seen, but they could exist. 280 00:18:10,189 --> 00:18:15,760 If primordial black holes are real then the universe is flooded with black holes. 281 00:18:15,762 --> 00:18:18,362 [Hopkins] The smallest coul have the mass of Mount Evere 282 00:18:18,397 --> 00:18:20,431 packed into the size of one atom. 283 00:18:20,433 --> 00:18:27,438 The biggest could be hundreds of thousands or millions of times the mass of the sun. 284 00:18:27,507 --> 00:18:30,775 [narrator] Stephen Hawking first suggested that primordial black holes 285 00:18:30,777 --> 00:18:35,713 could be dark matter back in the 1970s. 286 00:18:35,715 --> 00:18:40,384 The idea centers on what happened during that intangible momen 287 00:18:40,386 --> 00:18:45,890 13.8 billion years ago, the big-bang. 288 00:18:45,958 --> 00:18:50,461 Theory says that primordial black holes formed in the first fraction 289 00:18:50,463 --> 00:18:52,396 of a second of the early universe. 290 00:18:52,398 --> 00:18:55,099 It's that time between when the universe 291 00:18:55,101 --> 00:19:00,571 goes from a pinprick to this giant inflating ball of gas. 292 00:19:00,573 --> 00:19:03,741 [narrator] In these first moments of the universe's existence, 293 00:19:03,743 --> 00:19:07,145 matter is packed incredibly tightly. 294 00:19:07,180 --> 00:19:10,515 But it's not quite evenly spread. 295 00:19:10,517 --> 00:19:16,220 Even the tiniest fluctuation in density could trigger gravitational collapses. 296 00:19:17,490 --> 00:19:21,325 In other words, black holes would be forming everywhere, 297 00:19:21,861 --> 00:19:24,962 theoretically, in huge numbers. 298 00:19:25,732 --> 00:19:30,501 By the time one second has passed in our universe, 299 00:19:30,503 --> 00:19:33,271 you're already making black holes 300 00:19:33,339 --> 00:19:37,942 thousands, hundreds of thousands of times more massive than our sun. 301 00:19:37,944 --> 00:19:41,512 [narrator] The collective ma of these objects could be va 302 00:19:41,581 --> 00:19:46,617 but could they be 85% of the universe's matter? 303 00:19:46,619 --> 00:19:48,586 [Sutter] If primordial black holes really do exist 304 00:19:48,588 --> 00:19:53,658 there might be enough to explain the dark matter. 305 00:19:53,726 --> 00:20:00,765 [narrator] It's a tantalizin possibility, but there's on pretty big problem. 306 00:20:00,767 --> 00:20:03,734 For most scientists, the physics of the very early universe 307 00:20:03,736 --> 00:20:06,871 is incomplete and hard to trust. 308 00:20:06,873 --> 00:20:11,676 Generations of physicists dismissed primordial black holes 309 00:20:11,744 --> 00:20:17,014 as myths, fantasies, astrophysical unicorns, 310 00:20:17,016 --> 00:20:21,219 until that is, an earthshaki crash in space. 311 00:20:21,788 --> 00:20:24,422 May, 2019. 312 00:20:24,457 --> 00:20:27,458 A violent cosmic event rocks the USA. 313 00:20:27,460 --> 00:20:29,193 How violent? 314 00:20:29,195 --> 00:20:33,231 Well, the physical distance between Louisiana and Washington state 315 00:20:33,266 --> 00:20:39,570 is stretched by nearly the width of an atom which is bigger than it sounds. 316 00:20:39,572 --> 00:20:42,573 The Laser Interferometer Gravitational-Wave Observato 317 00:20:42,575 --> 00:20:45,977 detects this wobble in space time. 318 00:20:46,479 --> 00:20:51,582 This is the biggest gravitational wave event that LIGO has seen. 319 00:20:51,584 --> 00:20:56,554 [narrator] This cosmic disturbance seems to come from colliding black holes 320 00:20:56,556 --> 00:21:02,193 but crucially not the ordina dead star type. 321 00:21:02,228 --> 00:21:06,831 In this LIGO detection, one of the black holes is 85 solar masses. 322 00:21:06,833 --> 00:21:12,370 There's no way that a star could've made that black hole. 323 00:21:12,372 --> 00:21:15,406 [narrator] Physicists believ there's a range of masses 324 00:21:15,408 --> 00:21:20,878 where dying stars can't collapse into black holes. 325 00:21:20,880 --> 00:21:26,817 Instead, stars in this zone become insanely hot and rip themselves apart 326 00:21:26,819 --> 00:21:31,289 leaving nothing to crunch do into a black hole. 327 00:21:32,659 --> 00:21:35,726 Eighty-five solar masses sits right in the middle 328 00:21:35,795 --> 00:21:39,897 of this so called forbidden mass range. 329 00:21:39,899 --> 00:21:43,467 The black hole that LIGO detected can't be a dead star, 330 00:21:43,469 --> 00:21:47,638 but in theory it could be primordial. 331 00:21:47,640 --> 00:21:52,476 Could this discarded theory of dark matter be back in fashion? 332 00:21:52,478 --> 00:21:55,212 The LIGO detections come up and everyone says, 333 00:21:55,281 --> 00:21:57,748 "Oh, right, primordial black holes. 334 00:21:57,750 --> 00:22:00,618 Maybe we should pay more attention to that." 335 00:22:00,686 --> 00:22:03,054 Primordial black holes can be really appealing 336 00:22:03,056 --> 00:22:05,956 because they would solve the dark matter problem. 337 00:22:06,526 --> 00:22:09,694 But unfortunately, it's not that simple. 338 00:22:09,762 --> 00:22:13,798 The thing with flooding the universe with primordial black holes 339 00:22:13,800 --> 00:22:16,334 is that you expect a lot of collisions. 340 00:22:16,336 --> 00:22:18,636 And so LIGO shouldn't have seen one, 341 00:22:18,704 --> 00:22:23,040 it should have seen a thousand of these collisions and we don't. 342 00:22:24,110 --> 00:22:29,313 [narrator] Many scientists doubt what LIGO saw was a primordial black hole 343 00:22:30,283 --> 00:22:34,452 To them, these beasts remai fairytales of physics, 344 00:22:34,454 --> 00:22:38,489 red herrings in the quest for solid evidence of dark matter. 345 00:22:41,828 --> 00:22:43,861 Does dark matter exist? 346 00:22:43,863 --> 00:22:46,630 Or are we chasing shadows? 347 00:22:46,632 --> 00:22:49,467 Some scientists think it's not only real, 348 00:22:49,469 --> 00:22:52,770 but the dark matter is within our grasp, 349 00:22:52,772 --> 00:22:57,274 and that it's flying through our bodies right now 350 00:23:07,120 --> 00:23:11,389 [narrator] We think 85% of the universe's matter is dark. 351 00:23:12,558 --> 00:23:15,693 And yet, we've never found a speck of it. 352 00:23:15,695 --> 00:23:19,797 We can't prove dark matter exists. 353 00:23:19,832 --> 00:23:25,035 Regular matter is made up of everyday particles, like electrons and protons. 354 00:23:26,472 --> 00:23:31,409 Scientists wonder if dark matter is also a type of particle. 355 00:23:32,512 --> 00:23:34,712 One of the leading candidates for dark matter 356 00:23:34,714 --> 00:23:38,449 are these things called Weakly Interacting Massive Particles. 357 00:23:38,517 --> 00:23:41,652 They're massive particles like protons and electrons and things like that. 358 00:23:41,720 --> 00:23:44,922 But they don't interact well with normal matters, so they're weakly interacting. 359 00:23:44,924 --> 00:23:48,092 And they just have this name because it's awesome to call them WIMPs. 360 00:23:49,595 --> 00:23:51,962 [narrator] For decades, scientists have struggled 361 00:23:51,964 --> 00:23:55,966 to find these shy theoretical particles. 362 00:23:55,968 --> 00:23:59,069 [Oluseyi] The very first physics research I ever did in my life 363 00:23:59,071 --> 00:24:04,475 was about actually measuring directly dark matter particles, 364 00:24:04,477 --> 00:24:06,143 these so called WIMPs. 365 00:24:06,145 --> 00:24:10,681 And if they exist, then there will be a flux of millions of them 366 00:24:10,683 --> 00:24:13,851 through my hand right now, just by holding out right here. 367 00:24:13,853 --> 00:24:17,455 If dark matter is actually made of WIMPs, if these particles exist, 368 00:24:17,457 --> 00:24:21,525 then we're actually living basically in a sea of them. 369 00:24:21,527 --> 00:24:25,496 It surrounds and penetrates us and it bin the galaxy together. 370 00:24:26,732 --> 00:24:30,768 [narrator] WIMPs don't play by our rules. 371 00:24:30,770 --> 00:24:33,804 They barely interact with the world of regular matter, 372 00:24:33,806 --> 00:24:36,006 so they're hard to detect. 373 00:24:36,509 --> 00:24:38,776 But when they play with each other, 374 00:24:38,844 --> 00:24:45,216 sparks fly, intense flashes that we just might be able to see. 375 00:24:45,952 --> 00:24:49,887 [Plait] As the theory goes, WIMPs will self-annihilate. 376 00:24:49,889 --> 00:24:53,023 WIMP A and WIMP B get too close together, poof, 377 00:24:53,025 --> 00:24:55,326 they explode and they create gamma rays. 378 00:24:56,362 --> 00:25:01,098 [narrator] Gamma rays are high energy light, making them easy to spot. 379 00:25:03,536 --> 00:25:07,705 Scientists point their detectors at the cente of the Milky Way, 380 00:25:07,707 --> 00:25:12,943 where they believe the WIMP collision rate should be especially high. 381 00:25:13,012 --> 00:25:15,212 [Plait] We have a 4 million solar mass black hole there 382 00:25:15,214 --> 00:25:16,480 There are billions of stars there. 383 00:25:16,548 --> 00:25:20,751 That's where most of the mass of the galaxies is densest. 384 00:25:20,819 --> 00:25:24,455 So any WIMPs orbiting the galaxy will feel this natural attraction 385 00:25:24,457 --> 00:25:26,891 towards the center and fall toward it. 386 00:25:26,893 --> 00:25:29,793 [narrator] The Fermi Large Area Telescope scoured 387 00:25:29,862 --> 00:25:33,264 the center of our galaxy for more than 10 years. 388 00:25:33,666 --> 00:25:36,333 It detected lots of gamma rays, 389 00:25:36,335 --> 00:25:41,405 but scientists couldn't tel if they came from colliding WIMPs. 390 00:25:41,407 --> 00:25:45,009 The Galactic Center is a mess. It's like downtown of a city, right? 391 00:25:45,011 --> 00:25:47,478 That's where everything is, where all the hustle and bustle is. 392 00:25:47,480 --> 00:25:49,446 There are stars exploding there, 393 00:25:49,515 --> 00:25:51,882 just tons of stars, gas, magnetic fields, a black hole, 394 00:25:51,884 --> 00:25:53,651 a lot of sources of gamma rays, 395 00:25:53,653 --> 00:25:56,453 so it's very difficult to tease out the signal. 396 00:25:56,455 --> 00:25:59,056 [narrator] Downtown Milky Wa was a washout. 397 00:25:59,058 --> 00:26:01,725 So the scientists turned their attention to planets 398 00:26:01,794 --> 00:26:03,861 living in less noisy ZIP codes, 399 00:26:03,863 --> 00:26:07,965 where WIMP collisions should be easier to spot. 400 00:26:07,967 --> 00:26:10,634 One place where you might see evidence for WIMP collisions 401 00:26:10,636 --> 00:26:12,903 is actually the cores of exoplanets. 402 00:26:12,905 --> 00:26:19,143 Turns out exoplanets might be the best dark matter detector we have. 403 00:26:20,012 --> 00:26:22,880 You can use giant planets orbiting distant stars 404 00:26:22,882 --> 00:26:26,150 as laboratories to understand dark matter. 405 00:26:26,886 --> 00:26:30,521 [narrator] We know gravity should attract WIMPs. 406 00:26:30,589 --> 00:26:35,759 The more gravity, the more dark matter particl come together. 407 00:26:35,761 --> 00:26:39,964 Scientists suggest that WIMP congregate inside the cores 408 00:26:40,032 --> 00:26:42,800 of the Milky Way's largest gas planets. 409 00:26:42,802 --> 00:26:45,669 In these super-sized gas giants, 410 00:26:45,671 --> 00:26:51,408 WIMPs could collide, annihilate, and release gamma rays. 411 00:26:51,410 --> 00:26:53,477 If there are these WIMPs that are collecting the centers 412 00:26:53,479 --> 00:26:56,013 of mass of exoplanets, the annihilation of that dark matter 413 00:26:56,015 --> 00:26:57,881 can heat those exoplanets up. 414 00:26:57,883 --> 00:27:02,286 If you have a WIMP-heated exoplanet, and that's just fun to say, 415 00:27:02,354 --> 00:27:03,854 this thing is going to be warm, 416 00:27:03,856 --> 00:27:06,824 it's gonna be warmer than the heat of space, which is very cold. 417 00:27:06,826 --> 00:27:09,159 So what you need is an infrared telescope, 418 00:27:09,161 --> 00:27:11,695 something that sees an infrared light and is sensitive enough 419 00:27:11,697 --> 00:27:14,798 to be able to measure the temperatures of these things. 420 00:27:14,867 --> 00:27:20,037 [narrator] But a dedicated telescope like this won't launch until 2028. 421 00:27:21,741 --> 00:27:25,542 For some dark matter hunters that's too long to wait. 422 00:27:25,544 --> 00:27:29,480 They argue that WIMPs do have one characteristic 423 00:27:29,482 --> 00:27:33,150 that should allow us to detect them right here on Earth. 424 00:27:33,886 --> 00:27:37,821 The key to detecting WIMPs is in their name, it's the W-I. 425 00:27:37,823 --> 00:27:40,824 They're weakly interacting. They're not not interacting. 426 00:27:40,826 --> 00:27:43,594 They do interact, it's just very weak with matter. 427 00:27:43,629 --> 00:27:46,497 And that means that there are the rare occasions 428 00:27:46,499 --> 00:27:48,766 where it will smack into a particle of normal matter 429 00:27:48,834 --> 00:27:51,268 and then there are effects that we can observe. 430 00:27:52,772 --> 00:27:58,409 [narrator] Scientists in Gran Sasso in Central Italy watch for a spark of energy 431 00:27:58,444 --> 00:28:02,680 generated when a WIMP hits an atom of regular matter. 432 00:28:02,715 --> 00:28:06,517 Their detector, a tank of super cooled xenon 433 00:28:06,519 --> 00:28:11,488 built thousands of feet beneath the Earth's surface 434 00:28:11,490 --> 00:28:14,992 The beauty of putting this detector under a mountain is that you've got all of this 435 00:28:14,994 --> 00:28:16,727 rock and soil and everything else 436 00:28:16,729 --> 00:28:18,796 which is blocking a lot of background noise. 437 00:28:18,831 --> 00:28:20,931 When you're looking for a WIMP interaction, 438 00:28:20,933 --> 00:28:24,568 you're looking for something that's very rare and something very subtle, 439 00:28:24,570 --> 00:28:27,004 so you don't want other things going on. 440 00:28:27,039 --> 00:28:29,406 You don't want other particles coming in and messing up your experiment. 441 00:28:29,475 --> 00:28:33,844 These Weakly Interacting Massive Particles will pass right through that mountain, 442 00:28:33,912 --> 00:28:36,413 and then if they smack into a xenon atom, we can look at it and go, 443 00:28:36,415 --> 00:28:39,383 "Ah, that was a dark matter particle." 444 00:28:39,385 --> 00:28:44,922 [narrator] Detecting a WIMP could be definitive proof that dark matter exists. 445 00:28:44,990 --> 00:28:49,893 In 2020, the scientists spotted something in the results. 446 00:28:49,895 --> 00:28:52,296 But was it the elusive evidence 447 00:28:52,998 --> 00:28:56,200 or a ghost among the stars? 448 00:29:03,609 --> 00:29:06,744 [ambient music playing] 449 00:29:06,746 --> 00:29:09,279 [narrator] Scientists believ they can prove dark matter is real 450 00:29:09,315 --> 00:29:12,616 by detecting WIMPs. 451 00:29:12,618 --> 00:29:16,787 An experiment buried deep beneath an Italian mountain 452 00:29:16,789 --> 00:29:21,091 spotted unusual activity in a tank of regular matter 453 00:29:21,093 --> 00:29:22,893 pure liquid xenon. 454 00:29:22,895 --> 00:29:24,995 [popping] 455 00:29:26,465 --> 00:29:29,967 So a WIMP detector, like the XENON1T, 456 00:29:30,002 --> 00:29:33,904 waits for a little WIMP, tiny, tiny little particle 457 00:29:33,972 --> 00:29:36,740 to hit an atom of normal matter, 458 00:29:36,742 --> 00:29:38,742 and that creates a vibration. 459 00:29:38,744 --> 00:29:42,713 And we can see this entire block of xenon shake 460 00:29:42,781 --> 00:29:46,250 just a little bit from that little, subatomic collision. 461 00:29:47,453 --> 00:29:49,186 [narrator] The intensity of the vibration 462 00:29:49,188 --> 00:29:52,122 from the particle collision is critical. 463 00:29:52,124 --> 00:29:58,529 In theory, a WIMP striking a xenon atom should generat a powerful shock. 464 00:29:58,531 --> 00:30:03,400 The vibrations XENON1T detected were too weak. 465 00:30:05,037 --> 00:30:08,005 When a WIMP comes through, it smashes into the atom. 466 00:30:08,007 --> 00:30:11,508 It seemed like here something was just sort of rattling the electrons 467 00:30:11,510 --> 00:30:13,243 on the outside of the atom. 468 00:30:13,278 --> 00:30:15,579 So whatever is causing these detections was likely 469 00:30:15,581 --> 00:30:18,482 something much smaller than a WIMP. 470 00:30:18,484 --> 00:30:21,285 [Sutter] Let's take these results at face value 471 00:30:21,353 --> 00:30:23,921 It... If they're correct, it's telling us 472 00:30:23,989 --> 00:30:26,423 that the dark matter isn't a WIMP, 473 00:30:26,458 --> 00:30:28,392 but something much, much smaller 474 00:30:28,394 --> 00:30:30,327 and something much, much lighter. 475 00:30:31,363 --> 00:30:34,464 [narrator] The results sugge that what hit the xenon 476 00:30:34,466 --> 00:30:40,771 was actually a much smaller theoretical particl called an axion. 477 00:30:40,839 --> 00:30:44,141 [Bullock] Axions are really weird particles, incredibly light. 478 00:30:44,143 --> 00:30:47,911 In fact, almost zero mass is possible for an axion. 479 00:30:47,913 --> 00:30:50,514 An axion is no bigger 480 00:30:50,582 --> 00:30:55,452 than 150 billionth the size of an electron. 481 00:30:55,454 --> 00:31:02,726 Compared to a WIMP, an axion is like a soccer ball compared to our sun. 482 00:31:02,728 --> 00:31:05,529 [narrator] The sheer tinines of axions makes them seem 483 00:31:05,531 --> 00:31:08,065 like an unlikely candidate for dark matter. 484 00:31:09,735 --> 00:31:14,972 If dark matter is real, it makes up 85% of the matte in the universe. 485 00:31:18,010 --> 00:31:24,147 To account for all that mass we would need an almost unfathomable number of axion 486 00:31:24,950 --> 00:31:28,585 142 trigintillion of them, in fact. 487 00:31:28,587 --> 00:31:32,022 That's 140 with 93 zeros after it. 488 00:31:33,359 --> 00:31:37,828 If axions exist, space must swimming with them. 489 00:31:37,830 --> 00:31:42,432 They must be packed into every corner of the cosmos. 490 00:31:42,501 --> 00:31:46,003 When regular matter clumps together, it forms stars. 491 00:31:46,639 --> 00:31:49,606 So, to prove dark matter exists, 492 00:31:49,608 --> 00:31:52,442 maybe we should be looking for dark stars. 493 00:31:53,979 --> 00:31:56,213 There's no reason they can't exist. 494 00:31:56,215 --> 00:31:58,048 There's even a name for them 495 00:31:58,918 --> 00:32:00,384 Ghost stars. 496 00:32:01,553 --> 00:32:03,820 [Bullock] They're very weird objects. 497 00:32:03,822 --> 00:32:07,925 These ghost stars are like nothing we would ever see in the night sky. 498 00:32:07,927 --> 00:32:09,426 [narrator] We've never seen a ghost star. 499 00:32:09,428 --> 00:32:14,865 They are theoretical object made of hypothetical axions 500 00:32:14,867 --> 00:32:20,203 But in theory, ghost stars should form like any other star, 501 00:32:20,238 --> 00:32:22,539 pulled together by gravity. 502 00:32:22,541 --> 00:32:28,412 They would be gigantic, super dense objects floating through space. 503 00:32:28,414 --> 00:32:32,316 They could reach the mass of tens of millions of suns 504 00:32:32,818 --> 00:32:35,886 But because they are made of dark matter, 505 00:32:35,921 --> 00:32:40,691 ghost stars would produce no energy and emit no light 506 00:32:40,693 --> 00:32:45,662 They would be transparent to both light and matter. 507 00:32:45,664 --> 00:32:48,865 If you were right next to it, you wouldn't even notice it, right? 508 00:32:48,900 --> 00:32:52,469 If we sent a probe through it, it'd sail right through it 509 00:32:52,471 --> 00:32:55,906 uh, and once it passed through, it would be pulled back by its gravity. 510 00:32:55,941 --> 00:33:02,579 [narrator] 85% of the matte in our universe could consis of transparent orbs 511 00:33:02,581 --> 00:33:07,317 made of infinitesimally small, dark matter particles 512 00:33:08,053 --> 00:33:11,221 But do these invisible stars exist? 513 00:33:11,289 --> 00:33:13,457 The evidence is thin, but.. 514 00:33:13,459 --> 00:33:14,758 [tape rewinding] 515 00:33:14,760 --> 00:33:18,428 [narrator] Rewind back to th LIGO detection in 2019. 516 00:33:21,900 --> 00:33:25,168 The gravitational wave detector picked up the signa 517 00:33:25,170 --> 00:33:28,372 of two massive objects colliding. 518 00:33:28,941 --> 00:33:30,374 [pulsating explosion] 519 00:33:31,377 --> 00:33:34,978 We call the event GW190521. 520 00:33:34,980 --> 00:33:38,849 Most scientists agree this w a black hole collision. 521 00:33:38,851 --> 00:33:43,053 But could it have been clashing ghost stars? 522 00:33:43,789 --> 00:33:45,989 [Plait] If there are ghost stars out there 523 00:33:45,991 --> 00:33:47,724 and they can interact with each other gravitationally, 524 00:33:47,726 --> 00:33:49,893 they may collide. 525 00:33:49,895 --> 00:33:51,828 And when they do, they would emit gravitational waves 526 00:33:51,864 --> 00:33:54,931 and it would look a lot like two black holes colliding. 527 00:33:54,933 --> 00:34:00,337 In fact, it would look theoretically very much like GW190521. 528 00:34:01,673 --> 00:34:04,341 [narrator] One collision, two explanations. 529 00:34:05,911 --> 00:34:09,379 Primordial black holes or ghost stars, 530 00:34:10,816 --> 00:34:13,483 LIGO can't tell them apart. 531 00:34:13,485 --> 00:34:18,989 Do these ideas bring us closer to proving the existence of dark matter 532 00:34:18,991 --> 00:34:25,128 Or are we just hurtling further down a weird physics rabbit hole 533 00:34:25,130 --> 00:34:29,566 [Chiara Mingarelli] Primordial black holes, ghost stars, axions, 534 00:34:29,634 --> 00:34:32,135 this is all very exotic physics. 535 00:34:32,171 --> 00:34:36,740 We can't take for granted that any of this is real or that it's not real. 536 00:34:36,742 --> 00:34:37,607 We just don't know. 537 00:34:37,609 --> 00:34:40,544 Dark matter is irritating. 538 00:34:40,546 --> 00:34:43,747 [groans] We know it's out there. We see its effects, right? 539 00:34:43,782 --> 00:34:48,919 But we can't see the dark matter and that's frustrating. 540 00:34:48,987 --> 00:34:51,888 And it's like a lot of young fields in astronomy. 541 00:34:51,924 --> 00:34:56,259 We have way more ideas than we do hard observations. 542 00:34:57,463 --> 00:35:00,397 [narrator] We have ideas, we have theories, 543 00:35:00,399 --> 00:35:05,435 but without direct observations, we just can't back them up with solid proo 544 00:35:06,839 --> 00:35:11,308 The more we look, the harder it is to find dark matter. 545 00:35:12,544 --> 00:35:16,813 Maybe it's primordial black holes from the early universe. 546 00:35:16,815 --> 00:35:22,285 Maybe it's a sea of particle that flow right through us every day. 547 00:35:22,621 --> 00:35:28,625 Or maybe it's gigantic, transparent ghost stars. 548 00:35:28,627 --> 00:35:34,197 Perhaps it's the combined ma of Santa's sleigh and the Easter Bunny's baske 549 00:35:34,199 --> 00:35:39,436 Or maybe all our physics is based on questionable mat 550 00:35:49,548 --> 00:35:54,851 [narrator] 85% of the stuff in the universe is missing in action. 551 00:35:54,853 --> 00:35:58,221 The search for this dark matter looks hopeless. 552 00:35:59,691 --> 00:36:03,059 This problem of dark matter is really a tough one. 553 00:36:03,061 --> 00:36:04,160 Everything that we've predicted 554 00:36:04,162 --> 00:36:07,164 and then gone and looked for, we're not finding. 555 00:36:07,166 --> 00:36:09,699 It's starting to become a huge embarrassment. 556 00:36:09,701 --> 00:36:11,902 Surely something so fundamental 557 00:36:11,904 --> 00:36:13,803 to our cosmology should be detectable. 558 00:36:13,805 --> 00:36:15,572 And yet, it remains elusive. 559 00:36:17,976 --> 00:36:22,579 [narrator] We're stumbling blindly around the limits of our understanding. 560 00:36:22,581 --> 00:36:26,283 As of right now, there are zero direct observations. 561 00:36:27,619 --> 00:36:31,054 Maybe dark matter doesn't exist after all. 562 00:36:31,056 --> 00:36:33,890 Instead of searching for an invisible substance 563 00:36:33,892 --> 00:36:36,760 affecting the universe with its gravity, 564 00:36:36,762 --> 00:36:41,731 maybe it's gravity we don't quite understand. 565 00:36:41,733 --> 00:36:45,936 If you're looking in a galaxy and it's spinning way too quickly, 566 00:36:45,938 --> 00:36:50,040 either there's a new ingredient in the galaxy, 567 00:36:50,042 --> 00:36:52,576 like dark matter, that holds it all together. 568 00:36:52,578 --> 00:36:56,813 Or you're misunderstanding the laws of physics. 569 00:36:56,815 --> 00:36:58,715 [narrator] To describe the effects of gravity, 570 00:36:58,717 --> 00:37:04,321 we use the nearly 350-year-o math of Sir Isaac Newton. 571 00:37:05,924 --> 00:37:10,460 Maybe to explain the excess gravity we see in the universe, 572 00:37:10,462 --> 00:37:13,163 it's not extra matter we nee 573 00:37:13,465 --> 00:37:15,498 It's better math. 574 00:37:15,500 --> 00:37:17,601 Although we understand very well how gravity works 575 00:37:17,603 --> 00:37:19,869 here on Earth and in our Solar System, 576 00:37:19,871 --> 00:37:22,739 perhaps when you get up to galactic scales, 577 00:37:22,741 --> 00:37:25,575 it actually behaves just slightly differently. 578 00:37:25,577 --> 00:37:29,079 And if that were the case, you can kind of tweak that idea 579 00:37:29,081 --> 00:37:33,149 until it fits the data we see of how galaxies are spinning around 580 00:37:33,151 --> 00:37:36,019 without needing dark matter. 581 00:37:36,922 --> 00:37:38,388 [narrator] Questioning the math of a legend 582 00:37:38,390 --> 00:37:41,358 of physics might sound like sacrilege, 583 00:37:41,360 --> 00:37:45,595 but to solve the dark matter conundrum, it has been done. 584 00:37:45,597 --> 00:37:50,800 It's called Modified Newtonian Dynamics or MOND. 585 00:37:50,802 --> 00:37:56,306 Modeling galaxies with this math produces very different results. 586 00:37:57,743 --> 00:38:01,845 On its surface, MOND is not a bad idea. 587 00:38:01,847 --> 00:38:04,748 In the same way that we would normally program a computer 588 00:38:04,750 --> 00:38:07,584 to include dark matter in our simulations, 589 00:38:07,586 --> 00:38:10,487 you can take that out, and instead program it 590 00:38:10,489 --> 00:38:12,922 with a different law of gravity with MOND. 591 00:38:12,924 --> 00:38:16,826 And then, you can set up a kind of spinning mass of gas 592 00:38:16,828 --> 00:38:19,562 and it does seem to be possible with MOND 593 00:38:19,564 --> 00:38:23,166 to get things settled down and look a bit like a real galaxy. 594 00:38:25,971 --> 00:38:29,472 [narrator] Changing the law of gravity accurately recreates 595 00:38:29,474 --> 00:38:34,010 the super-fast spin astronomers see through their telescopes. 596 00:38:34,012 --> 00:38:37,480 No need for dark matter. It doesn't exist. 597 00:38:37,916 --> 00:38:39,149 Case closed? 598 00:38:39,584 --> 00:38:40,884 Not by a long shot. 599 00:38:40,886 --> 00:38:43,453 With anything bigger than a galaxy, 600 00:38:43,455 --> 00:38:46,990 this artificial physics breaks down. 601 00:38:47,793 --> 00:38:50,994 MOND does really well on galaxy scales, 602 00:38:50,996 --> 00:38:52,796 but when you zoom out and you go to larger 603 00:38:52,798 --> 00:38:55,098 and larger structures in our universe, 604 00:38:55,100 --> 00:38:59,169 like clusters of galaxies, big, big structure, 605 00:38:59,171 --> 00:39:03,273 you see that MOND by itself can't reproduce all of our observations. 606 00:39:03,275 --> 00:39:04,774 There's something missing. 607 00:39:04,776 --> 00:39:05,675 Dark matter. 608 00:39:05,944 --> 00:39:07,377 Dark matter, dark matter, dark matter. 609 00:39:07,379 --> 00:39:09,012 Dark matter. 610 00:39:09,014 --> 00:39:13,350 In MOND, you still have to invoke the existence of material you can't see. 611 00:39:13,985 --> 00:39:15,652 [Andrew Pontzen] It basically introduces 612 00:39:15,654 --> 00:39:17,854 some of its own dark matter as well, 613 00:39:17,856 --> 00:39:21,358 which kind of negates the point of having MOND in the first place. 614 00:39:24,429 --> 00:39:28,064 [narrator] MOND doesn't replace dark matter. 615 00:39:28,066 --> 00:39:31,935 The universe still needs something to hold it togethe 616 00:39:31,937 --> 00:39:33,837 We just don't know what it i 617 00:39:33,839 --> 00:39:37,240 But there are plenty of new ideas flying around. 618 00:39:38,510 --> 00:39:41,411 In my theory, the dark matter is a super fluid. 619 00:39:42,848 --> 00:39:45,915 [narrator] It's a radical new theory of dark matter, 620 00:39:45,984 --> 00:39:48,785 particles not acting individually, 621 00:39:48,787 --> 00:39:54,023 but flowing as one invisibl mass around the galaxies. 622 00:39:54,726 --> 00:39:58,628 A super fluid is like an ordinary fluid that flows, 623 00:39:58,630 --> 00:40:02,132 but in this case, it flows without any resistance or viscosity. 624 00:40:02,134 --> 00:40:05,435 If I pour honey, it will flow very slowly. 625 00:40:05,437 --> 00:40:06,736 It has high viscosity. 626 00:40:06,738 --> 00:40:10,740 A super fluid will just flo and never stop flowing. 627 00:40:10,742 --> 00:40:14,844 [narrator] As the super flui dark matter flows around the universe, 628 00:40:14,846 --> 00:40:21,117 eddies and waves form large enough to engulf entire galaxies. 629 00:40:21,119 --> 00:40:24,954 The gravity of the fluid holds the stars together. 630 00:40:25,991 --> 00:40:31,694 But like most theories on dark matter, there's no direct evidence. 631 00:40:31,696 --> 00:40:35,131 If these waves are on the size of galaxies, 632 00:40:35,133 --> 00:40:39,769 then we have to find detectors that can detect those types of huge waves. 633 00:40:39,771 --> 00:40:41,971 They don't exist at the moment. 634 00:40:42,908 --> 00:40:45,675 [narrator] Which brings us back to square one. 635 00:40:45,677 --> 00:40:48,611 We just can't prove that dark matter is real. 636 00:40:48,613 --> 00:40:51,714 Primordial black holes, ghost stars, WIMPs, 637 00:40:51,716 --> 00:40:55,985 a super fluid sloshing about the cosmos, 638 00:40:55,987 --> 00:40:59,055 or maybe we're just using the wrong math. 639 00:41:00,625 --> 00:41:02,859 What's your money on? 640 00:41:02,861 --> 00:41:07,363 If I had to wager $20 on what dark matter is. 641 00:41:07,699 --> 00:41:08,865 Hmm. 642 00:41:08,934 --> 00:41:12,702 I would never place money on what dark matter is. 643 00:41:12,704 --> 00:41:14,437 I just think we have no idea. 644 00:41:15,173 --> 00:41:20,109 My money is on dark matter itself is real, 645 00:41:20,111 --> 00:41:22,212 but it's not the whole picture. 646 00:41:22,214 --> 00:41:26,249 I would say left socks in dryers. 647 00:41:26,251 --> 00:41:30,720 I would say remote controls that fall into sofa cushions and disappear. 648 00:41:30,722 --> 00:41:32,088 I would love there to be 649 00:41:32,090 --> 00:41:35,792 dark matter, ghost stars, planets, even dark matter people. 650 00:41:35,794 --> 00:41:37,460 I'm going all black. 651 00:41:37,963 --> 00:41:43,032 I think no current ideas are correct. 652 00:41:43,034 --> 00:41:47,337 I think dark matter is something that we haven't thought of yet. 653 00:41:48,974 --> 00:41:50,507 [narrator] Does dark matter exist? 654 00:41:51,309 --> 00:41:52,942 Watch the space.