Dark matter has a lot of questions surrounding it. Not just for the lay community, but in the scientific community as well. So, I thought I'd devote a post to dark matter: what we think it is, what we think it isn't, and how we know it's there.
It's probably best to tell this story starting from the beginning. It's all about gravity. Quick physics 101 refresher: Gravity is the name given to the attractive force between two massive objects. In fact, the word comes from the Latin word gravitas meaning "weight." Mass + gravitational attraction = weight. In space, you are "weightless" but you still have mass, much the same way that a charged particle is charged even when it's not feeling a force by another charge.
The Mystery of the Missing Mass
Now we know a lot about gravity. Based on centuries of observational data, we know exactly how strong it is (to many decimal points) and we know exactly how the gravitational force acts over large distances and on very massive systems, like stars, galaxies and galaxy clusters.
So it was very surprising when observations of the motions of galaxies in distant galaxy clusters as well as gas in nearby galaxies and even the stars in our own Milky Way galaxy didn't make any sense. The motions were too fast. As I explained in my previous post on black holes, you can escape from a gravitational attraction by moving fast enough to get out of its gravity well. Well, it looked like all of these objects seemed to have velocities well beyond what would be needed to escape their larger structures... yet they all remained bound. Meaning that stars in the Milky Way are travelling fast enough that they should be shooting off into empty space, yet something was holding them in in place. There was some additional gravitational force keeping them locked in, but everywhere they looked, they couldn't account for the extra force.
Gravity comes from mass, so there was some extra mass somewhere that was unaccounted for. This became known as "the mystery of the missing mass." Further exploration showed similar results all over the visible universe. Since we couldn't seem to see this extra mass that we knew was out there, a theoretical exotic form of matter was proposed that only interacted with ordinary matter through gravitational influences. This extra mass therefore became known as "dark matter" since it didn't seem to partake in electromagnetic interactions. In retrospect, "invisible matter" or "transparent matter" would probably have been better since dark matter isn't actually "dark." It doesn't absorb light, it simply doesn't interact with it at all.
New models were made trying to account for this stuff and suddenly new light was shed on universe (pun intended). Observing the motion of galaxies, we're able to determine with good accuracy where the dark matter resides and, in fact, how much there needs to be in order to account for observations. In fact, we have calculated with good precision that dark matter outnumbers "ordinary" matter five to one. Also, there's very good evidence that without dark matter, large scale structures like galaxies would have never formed in the first place.
I discussed the cosmic microwave background in my gravitational waves post below. Way back when this was happening, the universe was mostly uniform. I'm saying "mostly" because small perturbations in matter density were already starting to form. Now this early soup of hot dense particles had an enormous amount of pressure pushing it to expand, and that pressure was mainly in the form of electromagnetic forces. If there were no dark matter, the ordinary matter in the universe would have expanded far enough away from each each other that by the time it cooled enough to clump, it would have been spread far too thin to clump. Stars would not have formed. Since dark matter only interacts via gravity, it could ignore this outward pressure and was able to clump early on, which effectively seeded the universe for structure formation. We owe our existence to dark matter.
So...what is it?
Well that's the $64 million question, isn't it? There are many theories, but no one really knows. The most studied candidates are various particles that have not been proven to exist. The most promising is the WIMP, which stands for "weakly interacting massive particle." WIMPs are theoretical particles that are part of a broader model known as "supersymmetry." WIMPs fit the bill pretty well. As the name suggests, they are predicted to not interact much with ordinary matter, except via their mass effects (i.e. gravity). Additionally, if WIMPs actually exist, predictions based on the theories would put their densities roughly where they would need to be to be a dark matter candidate. However, given their other predicted properties, particle physicists expect that if WIMPs existed, they should have been observed by now using our sophisticated detection equipment. So far, nada.
Various other candidates have been put forward, but so far, nothing seems to fit exactly the way we'd like them to. The most intriguing guess as to the nature of dark matter I'll address below.
The Halo and the Dark Disc
Where exactly is dark matter? It's all around us. Since dark matter seems to only interact with ordinary luminous matter through gravitation, they tend to end up in the same places: dark matter attracts ordinary matter and vice versa. All galaxies seem to have comparable densities of dark matter. But dark matter doesn't compact down into discs like ordinary matter in spiral galaxies since that kind of compaction requires that dark matter radiate. So instead, dark matter surrounds galaxies in a fairly uniform sphere called "the halo."
The halo accounts for the large scale structure seen in galaxies, galaxy clusters, collisions of galaxies (the famous example illustrating the dark matter halo is in the shape of the Bullet Cluster) and in gravitational lensing (how light bends around massive objects). However, small scale structure (i.e. within a galaxy) doesn't seem to line up with predictions all that well. This is where things get even more interesting. Some recent observations of the galactic core of the Milky Way show it to be much less dense than expected given current models of how dark matter should influence the structure, meaning that something else is going on. This led to the theory of "self-interacting" dark matter. So far, all we know is that dark matter only interacts with ordinary matter through gravity, but there's nothing saying that dark matter can't interact with itself through forces that only it experiences.
It turns out this isn't so far fetched. Plenty of particles that we know about exhibit this property: electrons don't experience the strong nuclear force (the force that binds protons and neutrons together), and neutrinos don't experience the electromagnetic force. So what's to say that dark matter particles don't have their own force that luminous matter doesn't experience?
However, if you throw an additional force into the mix, then the large scale structure predicted by dark matter goes kerflooey. But... if only some of the dark matter self-interacts, then it works. If that were the case, then some of this self-interacting dark matter could interact and radiate (not through electromagnetism, but through its own analogous dark matter equivalent) and compact down into a dark disc that aligns itself with the ordinary matter disc of the galaxy. This, however, is a logical leap. It presumes multiple sub-types of dark matter: those that interact, and those that don't.
Occam's Razor suggests that we shouldn't complicate things if we don't need to, but not exploring the possibility of multiple sub-types of dark matter is awfully geocentric. Assuming that all dark matter is only one thing is like assuming that all aliens will look exactly alike. Just because it's unknown doesn't mean it can't have its own rich diversity of dark-particles that our "ordinary" matter has. In fact, since dark matter outnumbers "ordinary" matter by five to one, shouldn't it really don the "ordinary" label and we be the more exotic of the two?
Taking this a step further into the science fiction category, you could imagine that if there were a full subset of dark-forces, you could have dark-stars, dark-planets, and even (theoretically) dark-life. Such beings could occupy the same space general space as us and yet be invisible to us (much like the premise of Isaac Asimov's The Gods Themselves).
The proof is in the pudding
Will we ever know what's going on? Yes. There are currently many many experiments underway to ascertain the true nature of dark matter. Whether these are particle accelerators, satellites or underground detectors, the whole world is just waiting for that tantalizing glimpse behind nature's veil. In fact, more answers will be coming later this year. The GAIA satellite is on a five-year mission to collect velocity measurements on a billion stars in the Milky Way, which could prove or disprove partially-interacting dark matter and the dark disc theory above. If a dark disc exists, then it would mostly align with the galactic disc, but the two would wobble around each other. As the dark disc passes through the galactic disc of ordinary matter, it would drag stars with it, creating characteristic patterns of star velocities around the galaxy. Such measurements would not only tell us if the dark disc exists, but the mass of the dark disc, what proportion of dark matter self-interacts, and even the strength of that interaction.
So while we don't have all of the answers, we do know a lot, and there's a lot in the works. This is a very exciting time in cosmology as we're on the edge of some incredible breakthroughs in the next few years. For those interested in more detailed reading, I highly recommend Lisa Randell's Dark Matter and the Dinosaurs.