Question: what is dark matter?

Clare Burrage answered on 14 Mar 2012:

When we look at other galaxies in our universe we can see them rotating, but the stars at the edges of the galaxies are travelling much faster than we think they should be, based on our theories of physics and all the matter (stars and gas) that we can see in the galaxy. To explain what we see we need either a new theory of gravity or to add some new matter that we can’t see into the galaxy. Most scientist now prefer the second option, we add some new matter into the galaxy and this is what we call dark matter. But we don’t know very much about what it is, other than that we can’t see it.
There are experiments trying to detect dark matter. They look for collisions between dark matter and ordinary particles, which would happen very rarely. We can’t see the dark matter, but we can see its effects on the particles it knocks into. These experiments are buried deep underground where things are very quiet and so the experiment can be very sensitive.