Question: what is dark matter?

Adam Tuff answered on 14 Mar 2011:

Now here is a question where I can happily say I don’t know! Dark matter is literally what it is matter that is so dark, we can’t actually see it. We have several different ideas to why this is.

One is that dark matter is MACHOs (MAssive, Compact Halo Objects) – these are things like dead stars that are huge, but because they are cool we can’t see them because they aren’t bright.

Another idea is that they are WIMPs (Weakly Interacting massive particles) – these are particles that, unlike the particles that make you and me, don’t react much in every day life. The sun produces countles trillions upon trillions of a WIMP called a neutrino – thousands upon thousands are passing through you right now as you read this, but they won’t interact at all! Because they don’t interact (or don’t interact much) it’s hard to detect them and measure them – even i they don’t have much mass, because there are so many of them, they could account for the extra mass we see in the galaxy.