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Dark matter: the mysterious stuff that's out there in the universe. There's five times more of it than normal matter, and we know an awful lot about it (including some powerful evidence that I didn't truly understand until working on this video), but we still don't really know what it is, which seems kind of paradoxical. So let's dive into how we know so much about something whose essence remains unknown. One of the first observations that made us think something else must exist beyond what we can see is that galaxies spin way too fast!
The outer stars in galaxies orbit faster than it seems like they should be able to based on the amount of matter we can actually observe with telescopes - they're going fast enough to get flung out of the galaxies! But they don't. So something must be keeping the stars from flying off - if you assume it's some sort of extra matter that we can't observe with telescopes and that gravitationally holds the stars in, there would need to be five times as much of this matter, and it would need to distributed in a sort of diffuse cloud enveloping the whole galaxy in
order to keep the too-fast-stars from flying off. The overly-fast rotation of the outer rims of galaxies is probably the most famous evidence for dark matter, and many people perhaps rightfully wonder if maybe we're just making observational mistakes or don't understand galaxies well enough, or gravity well enough, or something else, and maybe dark matter doesn't actually exist. But. There's a lot more evidence to contend with before you get too excited about your alternative theory of gravity! The second observation that tells us there's something more out there, is that gravitational lensing around galaxies and galaxy
clusters is much stronger than it should be! We've known since 1919 that gravity attracts light as well as matter, and so when light from a distant star or galaxy passes a super massive object to get to us, the light bends and we see an almost lens-like behaviour on a huge scale. By measuring the amount of gravitational lensing, we can calculate the total mass of whatever is causing the lensing and compare it with the mass of the matter that we directly observe. These observations show, once again, that visible matter in galaxy clusters only
accounts for a minority of the total mass that's required to create such strong lensing effects, and that the mass that we can't see must be much more spread out than the visible matter. Gravitational lensing can reveal even more complex cosmic oddities: we've found several cases where two huge galaxy clusters have collided - or more specifically, the intergalactic clouds of hot gas collided, because the galaxies and stars in them pass right by each other. Our observations of the light emitted tell us the intergalactic gas weighs more than the galaxies,
but the amount of gravitational lensing implies the galaxies weigh more than the gas. One of the most obvious ways this could work is if there's something massive travelling along with the galaxies - Pretty good evidence for something like… dark matter! The third observation that tells us some additional mass is out there is that the cosmic background radiation (the oldest light in the universe) is too uniform! This is oversimplifying, but just after the big bang, the universe was essentially a hot plasma of photons bouncing off of electrons and atomic nuclei, and the plasma gravitationally attracted itself into clumps of varying densities and pressures.
Eventually the universe cooled enough so electrons were captured by nuclei to become atoms, and the photons stopped bouncing off of them and just traveled away in all directions in straight lines. We still see these photons today when we look past stars and galaxies - but because of the initial clumping, there are slight variations in the average energies of the photons when we look in different directions: these variations have been measured super accurately over the last 30 years, and they are TOO uniform/they are NOT big enough. If the early universe just contained regular
matter, we should see almost twice as much variability as we do for the large scale noise, and we should also see fewer small scale details than we do. Only by including additional non-interacting matter in our cosmological models can we correctly predict the amount of gravitational clumping right after the big bang that lead to the different fluctuations in the cosmic background radiation that we actually see when we look in different parts of the sky. To be honest, I hadn't really appreciated until making this video how strong a piece of evidence
the cosmic background radiation is: our best cosmological model (which includes dark matter), is able to model the fluctuations of the cosmic background radiation in different parts of the sky with remarkable precision. And the evidence doesn't stop there… The fourth bit of observational evidence for something else out there is that the universe is way too structured for the amount of matter that we see. When we look way out into space, we see on a cosmic scale that the universe is a colossal network of filaments and walls made from superclusters of galaxies clumping together. Our best cosmological model can predict a universe clumping together into filaments the way ours is,
but only if we include more matter than we can see, and only in a specific way: it must be non-interacting - or dark - and it must be moving relatively slowly: moving slowly allows dark matter to gravitationally clump together into the halos that we think surround every galaxy and to provide the necessary attraction between the galaxies and superclusters to create filaments. So we have a TON of evidence that there must be something out there, and that whatever it is must have very particular characteristics: It has to be non-interacting (or very weakly interacting) both with itself and regular matter because so far we've only been able
to observe it indirectly via its gravitational effects on other things we can see. There has to be just the right amount of it to account for the unusual rotational behavior of galaxies, and the extra strong gravitational lensing around galaxy clusters, and the structure of the cosmic web, and the particular size and intensity of the variations in the cosmic background radiation in different parts of the sky. These criteria together rule out every single particle we know of: they all either interact with each other too much or travel way too
fast. If dark matter can't be explained by known particles, it's natural to wonder what it could be. So, here's a tour of five legitimate (and somewhat popular) possibilities for dark matter, though there are even more where these come from. Dark matter theory number 1: It's some sort of particle that simply doesn't interact with anything (other than by gravity). The most obvious solution is to hypothesize that dark matter is just another particle roughly like the ones we already know about, except that it doesn't interact with anything… at all. The only
way we could know it's there would be thanks to its interactions with other things via gravity, so there could be thousands or millions or trillions of dark matter particles per cubic meter all around us, going through you right now, and we'd never be able to detect them other than via the gravitational evidence we already have. Which would be sad and boring - cosmologists call it the "nightmare scenario"… but it's totally possible. The only way to know for sure, though, would be to be all knowing - I mean, would be to rule out the other options, so…
Here's dark matter theory number 2: what if it's a matter particle kind of like the ones we know of, but it only interacts with the known particles very, very, VERY weakly? A weakly (but non-negligibly!) interacting dark matter particle would make scientists happy, because then it might be possible to detect! The most popular type of hypothesized particle like this is the not-so-creatively named "Weakly Interacting Massive Particle" or, WIMP. The word "massive" here means it has some mass at all, we guess roughly around the masses of the known
particles, and "weakly interacting" means that its direct interactions with other particles could only happen via the weak nuclear force, or some new unknown force that's even weaker. What got people extra excited about WIMPs is that there are several problems with the standard model of particle physics, one of which could be explained by an idea called supersymmetry, which also coincidentally predicts the existence of a very weakly interacting particle with mass - a potential dark matter candidate. If we set up a sensitive enough experiment in a particle accelerator, or in a quiet enough cryogenic environment,
we could in theory detect a WIMP (whatever it is) bumping into an atomic nucleus or something, perhaps solving the standard model issues and the dark matter mystery all at once. We've been looking with many different types of experiments for something like a WIMP (and there are still others looking for supersymmetry), but so far, no measurements have conclusively detected anything… So what about dark matter theory number 3: what if it's really lightweight? Instead of hypothesizing particles that are roughly similar to the size of the ones we already know, maybe dark matter could be many times lighter (but still not massless):
One hypothesis is a class of particles called axions, which arise out of the same type of theoretical modeling that describes the Higgs boson, and which like WIMPs could potentially also solve a problem with the standard model of particle physics (though a different one from WIMPS. Axions were even named after a brand of washing detergent because they clean all your particle physics & cosmology problems away! Anyway, there are a variety of different axion-like theories, including "fuzzy" axions that are so incredibly light that their
quantum wavelengths becomes so large that they behave more like a blob of quantum fuzziness. To have the chance of detecting an axion (if they exist) you need to set up a very delicate experiment: an axion (in theory) can "scatter" off a strong electromagnetic field and turn into a photon with a frequency determined by the energy of the axion. These photons could be detected with a well-tuned resonant cavity, like trying to tune into a very weak signal with a radio. But so far we haven't detected any axions. So maybe instead, we can try: Dark matter theory number 4: what if it's really, really heavy. Like, a bunch of
large-scale astronomical bodies we can't see… The idea is that, rather than being a sea of tiny particles, dark matter might instead be a polka dot pattern of heavy, dense, dark objects we just haven't noticed yet looking out from planet Earth. The polka dots could be a number of things - Black Holes, cooled-off neutron stars, Red dwarfs, rogue planets, etc - and this idea is called "Massive Compact Halo Objects" also known as MACHOs - massive because they have masses on the same scale as stars, compact because of the polka dot pattern, and halo because they would explain the halos of inferred additional
matter around galaxies and galaxy clusters). Unlike WIMPs and axions, which at this point are entirely hypothetical, we DO know black holes and neutron stars exist, and the question instead becomes: could there be enough of them sneakily floating around to account for dark matter? We can get a sense of how many of these objects there are by looking at "gravitational microlensing:" when a heavy and dark object passes near a very bright object, the light is subtly bent inwards, and from our point of view we see a spike in the luminosity of the bright object - a bit like a magnifying glass passing in front of a light. Thousands of
these subtle flashes are observed every year as we look towards our galactic center, but we don't see enough to account for the necessary quantity of dark matter in and around our galaxy unless the objects are basically all large black holes. There's also the problem of the cosmic background radiation's uniformity - that uniformity means that only around 20 percent of all matter can be made from known particles, and the rest must be from non-interacting dark matter. Which rules out neutron stars and red dwarfs, AND it rules out black holes that formed over time from collapsing
stars or other regular matter. So if black holes are the solution to dark matter, they need to be black holes that formed directly during the big bang, called "primordial black holes." And the James Webb Space telescope may be seeing more primordial black holes than were expected… we'll have to wait and see whether there are enough to explain dark matter, or whether it'll turn out to be one of the other hypothetical particles (including a LOT more ideas we haven't gone into here). But it's also possible that particles of any sort are simply the wrong way to look at the dark matter problem, which brings us to… Dark matter theory number 5: maybe we just don't understand gravity well enough?
There are various proposed ideas for how "modified gravity" could fix our cosmic problems; one of them, called "Modified Newtonian Dynamics", postulates that in situations where the acceleration from gravity is very, very small (such as, conveniently, stars orbiting in the far outer portions of a galaxy), what if for low accelerations the attraction from gravity stopped decreasing as much as than the well-known 1/r^2 law, so that gravity was stronger than we'd otherwise expect? Stronger gravity could explain the movements of stars and galaxy
clusters without invoking any additional matter - that is, without any dark matter particles. And it does - modified gravity theories explain the overly-fast spinning of galaxies very well! But there's other evidence for dark matter (like the fluctuations in the cosmic background radiation, or gravitational lensing) that modified gravity has a harder time explaining. For example, remember the galaxy clusters where the gas collided but the stars just passed right through (along with extra matter inferred from gravitational lensing) - for those
colliding clusters, modified gravity theories can only explain a portion of the missing mass, but not all of it - some of it would still need to be made up of some other type of dark matter particles. Which brings us to the BONUS dark matter theory: dark matter could be a combination of different things! We humans like to find simple solutions to problems, and so it's in our nature to assume one single new discovery will solve the dark matter problem. But the reality is, it could be a mixture of some or all of the above: perhaps gravity does behave a bit differently and there are WIMPS; maybe there are both axions and
primordial black holes, each one only a portion of the dark matter pie. Maybe dark matter is a mixture of one of these ideas with something we haven't considered yet! OR… maybe dark matter IS the nightmare scenario: sad, boring, completely collisionless, undetectable and impossible to measure other than by its gravity, in which case we may never fully understand what dark matter is. This video is part of 50 by 50: Science Across America; a series of science videos across all 50 states, and I'm proudly representing Minnesota, which has a history of hunting for dark matter
in experiments located deep underground. The 2300-ft deep Soudan mine in Minnesota was an iron mine which operated from 1882 to 1962. It's now a state park (I remember getting to go down into the mine on a school trip when I was a kid!) and a science lab - after the mine closed, physicists started using it for sensitive physics experiments for its protection from cosmic rays. In making this video, I spoke with Professor Vuk Mandic, an astrophysicist at the University of Minnesota who was involved in the cryogenic Dark Matter Search (or CDMS) experiments at the Soudan mine. CDMS was looking for the type of weakly
interacting massive particles predicted by supersymmetry. If WIMPs exist, they would have no problem passing through the 700m of earth above the mine to reach the pure crystalline silicon or germanium wafers that served as particle detectors. Then hopefully the WIMP would collide with one of the atoms in the crystal lattice and cause a vibration big enough to detect. They had to cool the detector crystals to only a few thousandths of a degree above absolute zero to even have a chance of detecting any dark matter signals over the background thermal oscillations of the crystal. The cryogenic dark matter searches have since
moved to the even deeper mineshaft of the Sudbury neutrino laboratory in neighboring Canada, but Minnesota researchers are still involved! Prof. Mandic told me that one of the dark matter theories he's currently interested in is clouds of lightweight axion particles gravitationally bound to black holes - if that were the case, you could potentially learn things about dark matter from the gravitational waves of colliding black holes (which is what he now focuses on)! I'd also like to say a big thank you to Luna Zagorac for helping me better understand the
full breadth of dark matter and providing the initial inspiration for making this video, and to IMI and the 50 by 50: Science Across America project, for giving me the opportunity to represent Minnesota! Go check out science stories from the other states at 5050science.com!
