Question: What is dark matter?

Susan Cartwright answered on 15 Jun 2015:

I wish I knew!

In more detail:
– We have a huge amount of evidence from cosmology and astrophysics that there is about 6 times as much mass out there in the universe as we can account for with ordinary atomic matter. This evidence includes the orbital motion of stars and gas in galaxies (too fast to be accounted for by the visible mass of the galaxy), the orbital motion of galaxies in clusters of galaxies (also too fast), the distribution of X-ray emitting gas in galaxies (which traces the gravity of the cluster, and again requires more mass than we can account for), the gravitational bending of starlight around mass in deep space (again, requiring large amounts of invisible material) and the structure of the cosmic microwave background. All of these different probes are in remarkably good agreement: the energy density of the universe is made up of about 4% ordinary atoms, about 26% dark matter, and about 70% dark energy (which is probably unrelated to dark matter, and certainly even more mysterious).
– The dark matter cannot be ordinary atomic matter, because this would emit and/or absorb electromagnetic radiation (i.e. we would see it – if not in visible light, then in one of the other wavebands: X-rays, ultraviolet, infrared, microwaves, radio), and also because it would cause the early universe to make the wrong amount of deuterium and helium, and would create the wrong sort of patterns in the cosmic microwave background.
– Therefore, the dark matter must be a new type of particle (none of the particles in the Standard Model of particle physics has the required properties: neutrinos come closest, but their masses are too small and they move too quickly).
– There are many different proposed extensions to the Standard Model predicting new particles, some of which have the right properties to be potential dark matter candidates. The most popular, but by no means the only, candidates are Weakly Interacting Massive Particles (WIMPs) and axions (very light particles predicted by a theory which tries to explain why the differences between particles and antiparticles are confined to their weak interactions instead of also being seen in strong interactions).
– WIMPs might be produced at the Large Hadron Collider, if their masses are in the right range. Astrophysical WIMPs could also be detected by dedicated dark matter search experiments such as LUX, XENON and CDMS. Axions could be detected by carefully tuned magnetic detectors such as ADMX (Google all these names for more information!).
– Nobody has yet seen a confirmed signal: an experiment called DAMA/LIBRA has been claiming a signal for years, but it is not seen by any other experiment and – to be blunt – nobody believes them, because if the signal were real, other experiments should have seen something.

So the current state of play is that there is compelling indirect evidence for the existence of dark matter, and there are a number of proposed candidates, but so far none of these proposed candidates has been proven to exist at all, let alone to make up 85% of all the matter in the universe! So, at the moment, the answer to your question is “I don’t know – and neither does anyone else.”