“And what I wanted to do was, I wanted to explore problems and areas where we didn’t have answers. In fact, where we didn’t even know the right questions to ask.” -Donald Johanson
You can learn an awful lot about the Universe by asking it different questions than you asked about it previously. If all you ever used were your own senses, there would be an awful lot to learn, but you would be severely limited.
Even from the highest mountaintops, for example, you’d never be able to distinguish whether the Earth was round like a sphere or flat as a pancake, if all you used were your eyes. But by looking at the Earth on a larger scale than you could achieve otherwise, its roundness becomes both apparent and indisputable.
The same thing applies to the Universe, both on large scales and small. If you want to know what the overall structure is of the Universe, you have to look at it on the largest scales. Looking at individual galaxies or even large clusters of galaxies won’t get you there at all; if you want to know what your Universe looks like, you need to look at it on the largest and grandest of all scales, spanning billions of light years in all directions.
In the above image, still showing just a fraction of the Universe scanned and measured by the Sloan Digital Sky Survey, each pixel represents an entire galaxy. By measuring how galaxies cluster and clump together — how they are distributed throughout the Universe — we can determine what it takes to create a Universe that looks like ours.
What we learn, as you can go through in detail, is that the structure of the Universe requires that there be a type of matter in it that does not collide with either normal matter or with photons, that outnumbers our (normal) matter by a factor of five- or six-to-one, that don’t respond to either electric or magnetic fields, and that… frustratingly, can not be any of the known particles in the Universe!
This would be a very, very big problem under one condition:
If the known particles and laws of physics explained all of the observed phenomena in the Universe.
In other words, if there’s no new physics out there (beyond the standard model), then there’s no need for any new particles out there, and so, why would there be any dark matter? There simply wouldn’t be a strong motivation, not from an elementary physics standpoint.
And yet the opposite of that is also true: if there is physics out there that isn’t explained by the standard model, then there must be new types of particles out there! And if there are new particles out there, there are good candidates for this dark matter. You’ve probably heard of some of the speculations that abound:
For example, if there’s a symmetry of nature known as Supersymmetry (or SUSY, for short), then there ought to be twice as many fundamental particles as the ones we currently know about. Moreover, the lightest one is a perfect candidate for dark matter! Until we know what this particle’s properties are, however, we don’t know exactly what predictions to make as far as particle-particle interactions go.
While dark matter may or may not be supersymmetric in nature (many argue that SUSY may not even exist), this last part — that until we know what dark matter’s particle properties are, we don’t know what predictions to make for dark matter’s interactions — is generally true. But there are plenty of other ideas. Two more speculative ones, first, and then two definitive ones.
The electromagnetic, weak, and strong nuclear forces could all unify at some high energy, in what’s called a Grand Unified Theory, or GUT. One of the universal consequences of GUTs is that they all predict that protons will decay, and so that’s one of the things we look for. In many variants of GUTs, there are candidates for dark matter that emerge naturally.
Same case for extra dimensions; they may or may not exist, but if they do, then there are plenty of new particles and interactions that certainly exist, and one (or some) of them may make excellent dark matter candidates.
But those — supersymmetry, grand unification, and extra dimensions — are speculative ideas, and may not describe our Universe. But there are two observations that we have already made in the Standard Model that already cannot be explained by the particles and interactions we know today. This means there are new particles out there, yet undiscovered, that could easily solve the dark matter problem.
They are observed to have non-zero mass. In fact, all three types of neutrinos have non-zero mass, meaning there is new physics and there are new particles out there! Right-handed (or sterile) neutrinos could very easily make up the dark matter; we are searching for them as you read this! But perhaps the new physics that explains neutrinos isn’t also what explains dark matter. There’s another problem.
There are a couple of fundamental symmetries of nature that, at least in everyday life, seem pretty obvious. One is that the laws of physics in a mirror — where left and right are reversed — are the same as our normal laws of physics. (We call that Parity, or P-symmetry.) Another is that matter and anti-matter obey the same laws of physics. (We call that Charge Conjugation, or C-symmetry.) Most laws of physics that you know, like gravity and electromagnetism, always obey these symmetries.
According to the standard model, they have to; it’s coded into the physics. But these symmetries don’t exist for the nuclear (weak and strong) forces in the standard model. If I took something like a muon, reflected it in the mirror (applying P-symmetry), and replaced that image with an anti-muon (applying C-symmetry), I’d be testing whether the combination of CP-symmetry was a good one or not.
If it were a good symmetry, then if all the muons decayed with one orientation, all the anti-muons would decay with that specific, mirrored orientation. But they don’t, and so that CP-symmetry is violated. This is good for the Universe, because CP-violation is one of the necessary things to make more matter than anti-matter. But if it happens for an interaction like this — the Weak nuclear interaction — then it stands to reason that it should also happen for the strong nuclear force.
But it doesn’t! Why wouldn’t it?
The same reason this unicycle toy doesn’t tip over: there must be some sort of extra, hidden weight that provides extra balance, or in the particle’s case, crushes the amount of strong CP violation. Theoretically, the standard model allows you to violate both C and P together here, but we’ve looked, and to something like one part in a billion, we don’t see any. So something — and this means there’s new physics — has got to be forbidding it!
This outstanding problem, known as the Strong CP problem, is the second hint of new physics that must go beyond the standard model. And at least one class of solutions to it produces an outstanding dark matter candidate, known as the axion.
There’s definitely physics in this world that’s beyond the standard model, there’s definitely more to neutrinos than we know, and there’s definitely something stopping CP violation from occurring in the strong interactions. There may also be extra dimensions, grand unification, supersymmetry, or something even more exotic or surprising. But all of these possibilities require new particles, many of which make good dark matter candidates, and all of which have unknown particle parameters.
When you combine this information with our astrophysical knowledge of dark matter, you can see why I prefer the approach of using the astrophysics to try and reconstruct/determine some of the particle properties of dark matter, and try to guide us as to what we should look for. (No, really, I sometimes research that!)
We’ve got lots of options and lots of searches going, but there’s so much we don’t know about it at this point! Cross-sections, masses, reaction rates, lifetimes, etc., they’re all mysteries at this point. We may not know what dark matter is, exactly, but we’ve got lots of strong possibilities for what it could be, and some hints that simply can’t be ignored. We’re desperately trying to be able to detect it directly, and solve this mystery once and for all. Welcome to the cutting edge!