The mysterious dark matter is said to be like a cosmic glue that is holding together everything in this vast universe, yet it continues to play hide-and-seek with scientists.
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LearningTranscript
00:01Scientists believe there is a hidden substance deep in space that keeps the cosmos running.
00:08But is that substance real?
00:12We've never seen dark matter. It's completely invisible, but we know that it has to be there.
00:18Not only can you not see it, you couldn't really touch it or taste it or smell it, and yet it is all around us and affects everything that we do.
00:26After searching for decades, we still don't understand this inexplicable substance.
00:33We know dark matter is there because we feel its strong gravitational pull, but it just doesn't want to talk to us.
00:40There's evidence that dark matter makes up 85% of all the matter in the universe.
00:46We can see dark matter holding galaxies together and ripping other structures apart. We even see it bending light.
00:54Dark matter itself has been around since the beginning of the universe. Without dark matter, we wouldn't be here.
01:03But if you can't see dark matter, and if you can't touch it, does it really exist?
01:08Does it really exist?
01:24The Hyades Star Cluster.
01:26This family of 700 stars is 150 light years from Earth. At the scale of the universe, it's in our backyard.
01:37Hyades is actually close enough to Earth that you can see it with your naked eye.
01:40When you look up at the night sky, Hyades is in that V-shape in Taurus the bull.
01:47For most of its 650 million year lifetime, the Hyades enjoyed a peaceful existence.
01:55But something is breaking the calm.
01:58The Hyades Cluster is one of the most well-studied clusters of stars we have in the entire sky.
02:03And yet there's something very deeply mysterious going on.
02:07Two star tails extend from the cluster's center.
02:11They should be roughly equal.
02:14But one tail is hemorrhaging stars.
02:17Something is disrupting it. There's something exerting a force on it that's ripping stars out of their orbits.
02:23Something with immense gravitational pull has passed by the cluster and robbed it of stars.
02:33In order to be gravitationally pulling stars out of an object like Hyades, you need to have an incredibly massive structure as much as 10 million times the mass of the Sun.
02:42This monstrous cosmic mugger should still be visible.
02:48But when we point our telescopes to where it should be, that region is empty.
02:55There is nothing there. And I mean nothing.
03:00And not a little bit or something dark or something small, but there's literally nothing that we can see.
03:06We know something is out there, invisible and powerful.
03:13And whenever we witness these unseen assaults, a prime suspect gets called in.
03:19A phantom of physics.
03:22Dark matter.
03:25So what can we confidently say about this mysterious cosmic substance?
03:31It does not emit light. It does not reflect light.
03:34It does not absorb light.
03:37The only thing we know about dark matter is that it has gravity.
03:41We're not even really sure it's matter at all. It's just that that's the only thing we know that has gravity.
03:46We may not be able to see or touch dark matter, but we are very good at finding its fingerprints all over the universe.
03:54We can see dark matter's use of gravity to break and bind structures.
04:01And we've been spotting its handiwork for decades.
04:05Let's rewind back to 1933.
04:08Swiss-American physicist Fritz Zwicky tracks strange movements in a far-off collection of galaxies called the Coma Cluster.
04:19He knows he's not seeing the whole picture.
04:22Some galaxies are speeding around the cluster at inexplicably fast rates.
04:29Zwicky is looking at these galaxies, and if the only mass that was there were the other galaxies you can see,
04:35you would expect these galaxies to be moving at about 50 miles a second, then they would stay bound to each other and not fly apart.
04:41Instead, he sees them moving at 1,000 miles a second.
04:46At these velocities, galaxies should be flying off the cluster like sparks from fireworks.
04:54Zwicky realized there had to be extra stuff.
04:59In his words, dunkla materia.
05:03Dark matter.
05:05Dark matter.
05:06Dark matter.
05:07It becomes clear that Zwicky's Coma Cluster isn't an isolated case.
05:14Astronomers begin seeing the same dynamics within galaxies themselves.
05:20In systems governed exclusively by gravity, objects farthest away from the center would take the longest to complete an orbit.
05:29But in many galaxies, stars on the outside are orbiting at almost the same rate as those in the core.
05:37It's almost like a phonograph record.
05:39Every part of that record spins around like a solid disk.
05:43The stars are going too fast to stay bound to the gravity of the galaxy.
05:47They should just fly right off into space.
05:50Physicists come up with an explanation.
05:54Galaxies sit in a giant halo, or ball, of invisible dark matter.
05:59And it's that extra mass that allows the stars to turn fast, all the way out to the galactic rim.
06:07Think about actually taking a disk of dough and spinning it to make a pizza.
06:12The more you spin it, the more those outer regions go farther and farther away.
06:16Eventually the dough just goes flying everywhere.
06:19That's what would happen to a galaxy if it weren't for dark matter.
06:21But as you spin pizza dough and you spin it faster and faster, it does hold itself together because there's all this yummy gluten that's acting as a glue.
06:31Dark matter is the gluten of our universe.
06:35By calculating the mass needed to bind those speeding outer stars to the galaxy, physicists are able to estimate how much visible matter there is compared to dark matter.
06:47The results are staggering.
06:50All the stuff we thought existed was just maybe 15% of our universe.
06:55That's like if you go to a restaurant and realize there's a measly 15% tip, you know, that's all we are.
07:01We're not even the majority substance.
07:04We may not be able to see it, but dark matter makes up some 85% of all matter.
07:11Wherever we look, we can see its gravity having effects.
07:14It glues galaxies like our Milky Way together.
07:19And a close look, reveals dark matter, can also bend light itself.
07:27It's called gravitational lensing.
07:30A massive object can bend space and time.
07:34And light must follow the curves of that space and time.
07:38Gigantic clumps of any matter create a gravitational lens.
07:44Dark matter showed its space warping power in a trick it played with a gigantic explosion in a far off galaxy cluster.
07:54Supernova Refstall was first detected in November of 2014.
07:59Supernova Refstall is actually one of my favorite recent results in all of the astronomical literature.
08:08That result blew me away.
08:10So a star explodes, light is emitted in all directions, and some of it makes its way towards the Earth.
08:16So far, so good. This is very standard.
08:18So the flash appears, and then another flash appears.
08:25We see it again, and again, and again.
08:29We see the explosion go off in four parts of the sky, and then a year later, a fifth explosion goes off in a totally different part of the sky.
08:39What's going on?
08:41Analysis proves that these multiple explosions are the same supernova.
08:46But between this one dying star and our telescopes sits a giant mass of dark matter.
08:54A huge gravitational lens.
08:57What that means is that some of these rays of light will take much longer, more complicated paths through this region of space-time.
09:05The dark matter lens turns one supernova into a fireworks display, lasting an entire year.
09:15Dark matter affected the trajectory of light from this supernova so much that for some of those trajectories, it added a whole light year.
09:25It took a whole extra year for light to reach us.
09:28Something is very definitely out there, distorting our view of the cosmos.
09:36It's a potent clue that dark matter is real.
09:41Now, new evidence suggests that without it, we might not exist at all.
09:46The cosmos is filled with an unseen substance.
09:59Its mass even bends starlight.
10:03Gravitational lensing suggests dark matter holds our entire universe together.
10:09For decades, this specter of space has haunted us.
10:16We've never been able to pin it down.
10:18In 2021, an international team ran a virtual experiment to try to predict where dark matter should be by letting computers map out where we think it lives.
10:30Because we think we know how it behaves, we can model what it should be doing in supercomputer simulations.
10:38The team taught the computer how dark matter bends light.
10:43Then, applied computational power to 17,000 unexplored galaxies.
10:49The model created a dark matter map.
10:56I think a lot of people, when they imagine the universe on the larger scales, think it's sort of boring.
11:00Everything's uniform.
11:02But that's not what we see.
11:04What's amazing is that on the larger scales of the universe, we see a very particular pattern.
11:10When we zoom out, we see this magnificent structure, this cosmic web that's created by dark matter.
11:16The interweaving tendrils of dark matter stretch for thousands of light years across the cosmos.
11:24At the junctions where matter is concentrated, we find galaxies form, illuminating the dark scaffold.
11:33If dark matter exists, scientists believe it makes up 85% of the matter in the universe, and also controls the remaining 15%, regular matter, like stars, planets, us.
11:49If they're right, dark matter played a critical role in actually building the universe we see today.
11:592021.
12:05Astronomers using the SkyMapper Observatory in Australia train specialist optics on a dwarf galaxy called Tucana 2.
12:15The SkyMapper's filters split up the starlight into a spectrum of wavelengths, revealing some very ancient light.
12:24One of the best clocks that we can put on the universe is the progress of chemistry, right?
12:34The buildup of more complex elements over time.
12:38Stars are nothing if not factories of chemical complexity.
12:43They slam particles together and create heavier elements, right, through a process called fusion.
12:48The later the generation of star, the more chemically complex it is.
12:53Tucana 2's spectral signature reveals its stars contain very few of these heavy, complex elements.
13:01A clue that lets astrophysicists calculate the age of the galaxy.
13:07These are very, very old stars from the very early days of the universe, when the gas in the universe was not that chemically complex.
13:15Tucana 2 might be one of the oldest known structures that we can see in our local universe.
13:23It could be as old as 13 billion years.
13:26Almost as old as the universe itself.
13:30This grand old lady of a galaxy is a tiny thing, barely 3000 stars.
13:35And yet, way out on her galactic rim, stars hurtle around at breakneck speed.
13:45When you look at the mass of this ultra faint dwarf galaxy, it only has a few thousand times the mass of the sun.
13:52That's really small.
13:54And at the speed it's moving, it should fly apart.
13:56Tucana 2 doesn't break up because it's glued together.
14:00Apparently, by an incredible amount of dark matter.
14:05When you look at a galaxy like our Milky Way, it's about 85% dark matter, which is a lot.
14:10But with Tucana 2, it's more like 99%.
14:14Tucana 2 is old, among the oldest galaxies in the universe, and it is packed full of dark matter.
14:21Simulations suggest this dark matter played a key role in shaping Tucana 2 and other very early galaxies right from the beginning.
14:32Gathering regular matter into clumps and building the first galaxies.
14:38The importance of dark matter really can't be overstated.
14:42It has actually controlled the way matter has evolved since the beginning of the universe.
14:46It brings matter together.
14:49You need this underlying structure of dark matter to make it all happen.
14:53Scientists think that for billions of years, as the early universe grew, dark matter called the shots.
15:01Without its gravity, structures like the Milky Way wouldn't have formed.
15:06We've seen dark matter's light bending effects.
15:12We've even deduced where it should be.
15:15Dark matter really does appear to exist.
15:19But this evidence is indirect, circumstantial.
15:23To get conclusive proof that dark matter exists, don't we need to find some?
15:29If we could find a lump of dark matter, that would be one of the greatest discoveries in all of nature, in all of our history, right?
15:40Because we would understand one of the most fundamental components for how our universe works.
15:45Dropping the title, they love that.
15:48It's time to hunt for dark matter itself.
15:52Could it be hiding in the darkest place of all?
15:56Black holes.
16:00Scientists believe an invisible substance is pulling the strings in our universe.
16:16But until we see it, sense it, perhaps even touch it, dark matter is just a theory.
16:22Sometimes, though, ideas dreamed up by scientists come true.
16:29Like black holes.
16:32Once the stuff of science fiction and children's nightmares, black holes today are confirmed reality.
16:41So black holes and dark matter have a ton of similarities, right?
16:44You know, an unseen collection of matter that creates an enormous gravitational field. Check.
16:49It bends light and causes gravitational lensing. Check.
16:54Test the boundaries of known physics. Check.
16:58It seems crazy to even ask, but could our search for dark matter end in an idea more than a hundred years old?
17:06Could dark matter be black holes?
17:11Black holes appear when stars explode and their remaining mass crunches down into a sphere so dense even light can't escape its gravity.
17:23But that's where the black hole dark matter theory stumbles.
17:27We know that black holes happen. We know how they form. And we also know that there's nowhere near enough of them to be dark matter.
17:35Not enough stars have lived and died in the history of the universe to create 85% of the matter in it.
17:43If dark matter is made up of black holes, they would have to be an entirely new type.
17:49It's possible that these black holes are of a type that we've never seen before. They could be primordial black holes.
18:01Primordial black holes are an idea, a theoretical concept at this point that we've never seen, but they could exist.
18:09If primordial black holes are real, then the universe is flooded with black holes.
18:16The smallest could have the mass of Mount Everest packed into the size of one atom.
18:21The biggest could be hundreds of thousands or millions of times the mass of the sun.
18:27Stephen Hawking first suggested that primordial black holes could be dark matter back in the 1970s.
18:35The idea centers on what happened during that intangible moment 13.8 billion years ago, the Big Bang.
18:46Theory says that primordial black holes formed in the first fraction of a second of the early universe.
18:53It's that time between when the universe goes from a pinprick to this giant inflating ball of gas.
18:59In these first moments of the universe's existence, matter is packed incredibly tightly, but it's not quite evenly spread.
19:10Even the tiniest fluctuations in density could trigger gravitational collapses.
19:16In other words, black holes would be forming everywhere.
19:22Theoretically, in huge numbers.
19:25By the time one second has passed in our universe, you're already making black holes thousands, hundreds of thousands of times more massive than our sun.
19:37The collective mass of these objects could be vast, but could they be 85% of the universe's matter?
19:47If primordial black holes really do exist, there might be enough to explain the dark matter.
19:53The dark matter.
19:54It's a tantalizing possibility, but there's one pretty big problem.
20:01For most scientists, the physics of the very early universe is incomplete and hard to trust.
20:07Generations of physicists dismissed primordial black holes as myths, fantasies, astrophysical unicorns, until that is an earth-shaking crash in space.
20:22May 2019.
20:25A violent cosmic event rocks the USA.
20:28How violent?
20:29Well, the physical distance between Louisiana and Washington state is stretched by nearly the width of an atom, which is bigger than it sounds.
20:40The Laser Interferometer Gravitational Wave Observatory detects this wobble in space-time.
20:47This is the biggest gravitational wave event that LIGO has seen.
20:51This cosmic disturbance seems to come from colliding black holes, but crucially, not the ordinary dead star type.
21:02In this LIGO detection, one of the black holes is 85 solar masses.
21:08There's no way that a star could have made that black hole.
21:13Physicists believe there's a range of masses where dying stars can't collapse into black holes.
21:21Instead, stars in this zone become insanely hot and rip themselves apart, leaving nothing to crunch down into a black hole.
21:3285 solar masses sits right in the middle of this so-called forbidden mass range.
21:40The black hole that LIGO detected can't be a dead star, but in theory, it could be primordial.
21:47Could this discarded theory of dark matter be back in fashion?
21:53The LIGO detections come up and everyone says, oh, right, primordial black holes.
21:58Maybe we should pay more attention to that.
22:02Primordial black holes can be really appealing because they would solve the dark matter problem.
22:07But unfortunately, it's not that simple.
22:09The thing with flooding the universe with primordial black holes is that you expect a lot of collisions.
22:17And so LIGO shouldn't have seen one.
22:20It should have seen a thousand of these collisions.
22:22And we don't.
22:23Many scientists doubt what LIGO saw was a primordial black hole.
22:30To them, these beasts remain fairy tales of physics.
22:35Red herrings in the quest for solid evidence of dark matter.
22:38Does dark matter exist or are we chasing shadows?
22:47Some scientists think it's not only real, but the dark matter is within our grasp and that it's flying through our bodies right now.
22:57We think 85% of the universe's matter is dark.
23:13And yet we've never found a speck of it.
23:16We can't prove dark matter exists.
23:20Regular matter is made up of everyday particles like electrons and protons.
23:25Scientists wonder if dark matter is also a type of particle.
23:32One of the leading candidates for dark matter are these things called weakly interacting massive particles.
23:40They're massive particles like protons and electrons and things like that, but they don't interact well with normal matter.
23:45So they're weakly interacting.
23:46They just have this name because it's awesome to call them wimps.
23:50For decades, scientists have struggled to find these shy, theoretical particles.
23:56The very first physics research I ever did in my life was about actually measuring directly dark matter particles, these so-called wimps.
24:06And if they exist, then there would be a flux of millions of them through my hand right now just by holding it out right here.
24:15If dark matter is actually made of wimps, if these particles exist, then we're actually living basically in a sea of them.
24:20It surrounds and penetrates us and it binds the galaxy together.
24:27Wimps don't play by our rules.
24:31They barely interact with the world of regular matter, so they're hard to detect.
24:35But when they play with each other, sparks fly, intense flashes that we just might be able to see.
24:46As the theory goes, wimps will self-annihilate.
24:50If wimp A and wimp B get too close together, poof, they explode and they create gamma rays.
24:55Gamma rays are high energy light, making them easy to spot.
25:04Scientists point their detectors at the center of the Milky Way, where they believe the wimp collision rate should be especially high.
25:13We have a four million solar mass black hole there. There are billions of stars there.
25:18That's where most of the mass of the galaxy is densest.
25:21So any wimps orbiting the galaxy will feel this natural attraction towards the center and fall toward it.
25:27The Fermi Large Area Telescope scoured the center of our galaxy for more than 10 years.
25:34It detected lots of gamma rays, but scientists couldn't tell if they came from colliding wimps.
25:42The galactic center is a mess. It's like downtown of a city, right?
25:46That's where everything is, where all the hustle and bustle is.
25:48There are stars exploding there, just tons of stars, gas, magnetic fields, a black hole, a lot of sources of gamma rays.
25:55So it's very difficult to tease out the signal.
25:57Downtown Milky Way was a washout.
26:00So the scientists turned their attention to planets living in less noisy zip codes, where wimp collisions should be easier to spot.
26:08One place where we might see evidence for wimp collisions is actually the cores of exoplanets.
26:13Turns out exoplanets might be the best dark matter detector we have.
26:20You can use giant planets orbiting distant stars as laboratories to understand dark matter.
26:26We know gravity should attract wimps. The more gravity, the more dark matter particles come together.
26:36Scientists suggest that wimps congregate inside the cores of the Milky Way's largest gas planets.
26:43In these supersized gas giants, wimps could collide, annihilate, and release gamma rays.
26:51If there are these wimps that are collecting the centers of massive exoplanets, the annihilation of that dark matter can heat those exoplanets up.
26:58If you have a wimp-heated exoplanet, and that's just fun to say, this thing is going to be warm.
27:05It's going to be warmer than the heat of space, which is very cold.
27:08So what you need is an infrared telescope, something that sees an infrared light and is sensitive enough to be able to measure the temperatures of these things.
27:15But a dedicated telescope like this won't launch until 2028.
27:19For some dark matter hunters, that's too long to wait.
27:26They argue that wimps do have one characteristic that should allow us to detect them right here on Earth.
27:34The key to detecting wimps is in their name. It's the WI.
27:39They're weakly interacting. They're not not interacting. They do interact. It's just very weak with matter.
27:44And that means that there are the rare occasions where it will smack into a particle of normal matter, and then there are effects that we can observe.
27:52Scientists in Grand Sasso in central Italy watch for a spark of energy generated when a wimp hits an atom of regular matter.
28:03Their detector? A tank of supercooled xenon built thousands of feet beneath the Earth's surface.
28:10The beauty of putting this detector under a mountain is that you've got all of this rock and soil and everything else, which is blocking a lot of background noise.
28:20When you're looking for a wimp interaction, you're looking for something that's very rare and something very subtle.
28:26So you don't want other things going on. You don't want other particles coming in and messing up your experiment.
28:30These weakly interacting massive particles will pass right through that mountain, and then if they smack into a xenon atom, we can look at it and go, ah, that was a dark matter particle.
28:40Detecting a wimp could be definitive proof that dark matter exists.
28:46In 2020, the scientists spotted something in the results.
28:50But was it the elusive evidence or a ghost among the stars?
29:07Scientists believe they can prove dark matter is real by detecting wimps.
29:12An experiment buried deep beneath an Italian mountain spotted unusual activity in a tank of regular matter, pure liquid xenon.
29:24So a wimp detector, like the xenon 1T, waits for a little wimp, tiny, tiny little particle to hit an atom of normal matter.
29:38And that creates a vibration.
29:40And we can see this entire block of xenon shake just a little bit from that little subatomic collision.
29:47The intensity of the vibration from the particle collision is critical.
29:52In theory, a wimp striking a xenon atom should generate a powerful shock.
29:59The vibrations xenon 1T detected were too weak.
30:05When a wimp comes through, it smashes into the atom.
30:09It seemed like here something was just sort of rattling the electrons in the outside of the atom.
30:14So whatever is causing these detections was likely something much smaller than a wimp.
30:18Let's take these results at face value.
30:22If they're correct, it's telling us that the dark matter isn't a wimp, but something much, much smaller and something much, much lighter.
30:32The results suggest that what hit the xenon was actually a much smaller theoretical particle called an axion.
30:40Axions are really weird particles, incredibly light. In fact, almost zero mass is possible for an axion.
30:49An axion is no bigger than 150 billionth the size of an electron.
30:57Compared to a wimp, an axion is like a soccer ball compared to our sun.
31:02The sheer tininess of axions makes them seem like an unlikely candidate for dark matter.
31:09If dark matter is real, it makes up 85% of the matter in the universe.
31:15To account for all that mass, we would need an almost unfathomable number of axions.
31:25140 trigontillion of them, in fact. That's 140 with 93 zeros after it.
31:32If axions exist, space must be swimming with them. They must be packed into every corner of the cosmos.
31:43When regular matter clumps together, it forms stars.
31:47So, to prove dark matter exists, maybe we should be looking for dark stars.
31:53There's no reason they can't exist. There's even a name for them. Ghost stars.
32:02They're very weird objects. These ghost stars are like nothing we would ever see in the night sky.
32:08We've never seen a ghost star. They are theoretical objects made of hypothetical axions.
32:14But, in theory, ghost stars should form like any other star, pulled together by gravity.
32:24They would be gigantic, super dense objects floating through space.
32:29They could reach the mass of tens of millions of suns.
32:34But because they are made of dark matter, ghost stars would produce no energy and emit no light.
32:41They would be transparent to both light and matter.
32:46If you were right next to it, you wouldn't even notice it, right?
32:50If we sent a probe through it, it would sail right through it.
32:53And once it passed through, it would be pulled back by its gravity.
32:5785% of the matter in our universe could consist of transparent orbs
33:03made of infinitesimally small dark matter particles.
33:07But, do these invisible stars exist?
33:12The evidence is thin, but...
33:15Rewind back to that LIGO detection in 2019.
33:22The gravitational wave detector picked up the signal of two massive objects colliding.
33:29We call the event GW 190521.
33:35Most scientists agree this was a black hole collision.
33:40But, could it have been clashing ghost stars?
33:43If there are ghost stars out there, and they can interact with each other gravitationally, they may collide.
33:50And when they do, they would emit gravitational waves.
33:53And it would look a lot like two black holes colliding.
33:56In fact, it would look, theoretically, very much like GW 190521.
34:01One collision, two explanations.
34:05Primordial black holes, or ghost stars?
34:10LIGO can't tell them apart.
34:13Do these ideas bring us closer to proving the existence of dark matter?
34:19Or, are we just hurtling further down a weird physics rabbit hole?
34:25Primordial black holes, ghost stars, axions.
34:31This is all very exotic physics.
34:33We can't take for granted that any of this is real, or that it's not real.
34:37We just don't know.
34:39Dark matter is irritating.
34:42Oh, we know it's out there. We see its effects, right?
34:45But we can't see the dark matter.
34:48And that's frustrating.
34:50And it's like a lot of young fields in astronomy.
34:52We have way more ideas than we do hard observations.
34:58We have ideas.
35:00We have theories.
35:02But without direct observations, we just can't back them up with solid proof.
35:07The more we look, the harder it is to find dark matter.
35:13Maybe it's primordial black holes from the early universe.
35:17Maybe it's a sea of particles that flow right through us every day.
35:23Or maybe it's gigantic, transparent, ghost stars.
35:29Perhaps it's the combined mass of Santa's sleigh and the Easter Bunny's basket.
35:34Or maybe all our physics is based on questionable math.
35:4085% of the stuff in the universe is missing in action.
35:55The search for this dark matter looks hopeless.
35:58This problem of dark matter is really a tough one.
36:03Everything that we've predicted and then gone and looked for, we're not finding it.
36:08It's starting to become a huge embarrassment.
36:10Surely something so fundamental to our cosmology should be detectable.
36:15And yet, it remains elusive.
36:16It remains elusive.
36:19We're stumbling blindly around the limits of our understanding.
36:23As of right now, there are zero direct observations.
36:28Maybe dark matter doesn't exist after all.
36:32Instead of searching for an invisible substance affecting the universe with its gravity,
36:38maybe its gravity we don't quite understand.
36:41If you're looking at a galaxy and it's spinning way too quickly,
36:47either there's a new ingredient in the galaxy like dark matter that holds it all together,
36:54or you're misunderstanding the laws of physics.
36:58To describe the effects of gravity, we use the nearly 350-year-old math of Sir Isaac Newton.
37:05Maybe, to explain the excess gravity we see in the universe,
37:11it's not extra matter we need.
37:14It's better math.
37:16Although we understand very well how gravity works here on Earth and in our solar system,
37:21perhaps when you get up to galactic scales, it actually behaves just slightly differently.
37:27And if that were the case, you can kind of tweak that idea until it fits the data we see of how galaxies are spinning around
37:33without needing dark matter.
37:37Questioning the math of a legend of physics might sound like sacrilege,
37:42but to solve the dark matter conundrum, it has been done.
37:47It's called Modified Newtonian Dynamics, or MOND.
37:52Modeling galaxies with this math produces very different results.
37:57On its surface, MOND is not a bad idea.
38:02In the same way that we would normally program a computer to include dark matter in our simulations,
38:08you can take that out and instead program it with a different law of gravity, with MOND.
38:14And then you can set up a kind of spinning mass of gas,
38:18and it does seem to be possible with MOND to get things that settle down and look a bit like a real galaxy.
38:23Changing the law of gravity accurately recreates the super-fast spin astronomers see through their telescopes.
38:34No need for dark matter. It doesn't exist.
38:38Case closed? Not by a long shot.
38:42With anything bigger than a galaxy, this artificial physics breaks down.
38:48MOND does really well on galaxy scales,
38:50but when you zoom out and you go to larger and larger structures on our universe,
38:56like clusters of galaxies and big, big structure,
39:00you see that MOND by itself can't reproduce all our observations.
39:04There's something missing.
39:05Dark matter.
39:07Dark matter, dark matter, dark matter.
39:09Dark matter.
39:11In MOND, you still have to invoke the existence of material you can't see.
39:15It basically introduces some of its own dark matter as well,
39:19which kind of negates the point of having MOND in the first place.
39:25MOND doesn't replace dark matter.
39:29The universe still needs something to hold it together.
39:32We just don't know what it is.
39:34But there are plenty of new ideas flying around.
39:37In my theory, the dark matter is a superfluid.
39:42It's a radical new theory of dark matter.
39:47Particles, not acting individually,
39:50but flowing as one invisible mass around the galaxies.
39:55A superfluid is like an ordinary fluid that flows,
39:59but in this case, it flows without any resistance or viscosity.
40:03If I pour honey, it will flow very slowly.
40:06It has high viscosity.
40:08A superfluid will just flow and never stop flowing.
40:12As the superfluid dark matter flows around the universe,
40:16eddies and waves form, large enough to engulf entire galaxies.
40:22The gravity of the fluid holds the stars together.
40:25But, like most theories on dark matter, there's no direct evidence.
40:32If these waves are on the sides of galaxies,
40:36then we have to find detectors that can detect those types of huge waves.
40:41They don't exist at the moment.
40:43Which brings us back to square one.
40:47We just can't prove that dark matter is real.
40:50Primordial black holes, ghost stars,
40:52wimps, a superfluid sloshing about the cosmos.
40:57Or, maybe we're just using the wrong math.
41:00What's your money on?
41:03If I had to wager $20 on what dark matter is?
41:08Hmm.
41:10I would never place money on what dark matter is.
41:13I just think we have no idea.
41:15My money is on dark matter itself is real, but it's not the whole picture.
41:23I would say left socks and dryers.
41:27I would say remote controls that fall into sofa cushions and disappear.
41:32I would love there to be dark matter ghost stars, planets, even dark matter people.
41:37I'm going all black.
41:39I think no current ideas are correct.
41:44I think dark matter is something that we haven't thought of yet.
41:49Does dark matter exist?
41:52Watch this space.
41:53I think dark matter is something that we have like точ a while.
41:5518 years ago.
41:57A lot of ideas will mitigate that change.
41:58A lot of creativity will not be much enemigo.
42:0122 years ago.
42:02Yes.
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