Question: What is dark matter?

Arttu Rajantie answered on 14 Jun 2011:

First of all, dark matter is stuff that we cannot observe directly but which seems to be present in galaxies because otherwise they would rotate much more slowly. There are also many other observations that we can only explain if we assume that there is some unobservable matter in the universe. We can even make maps of how the dark matter is distributed in the universe by looking at the way its gravitational field bends light.

Some of this dark matter could be just gas, or even planets like the Earth, because after all, we can only see stars. However, from other observations we can calculate how much normal matter there is in the universe, i.e., matter made of atoms, and it turns out that that is not enough. Therefore most of the dark matter have to be some new unknown particles.

The only thing we really know about the dark matter particles is that they have to be stable, because otherwise they would have already decayed. The standard model of particle physics cannot explain dark matter, but if supersymmetry exists, as many particle physicists believe, then there are stable particles that could be dark matter. In that case the LHC should be able to find them and measure their properties.