Question: what is dark matter

Steve Marsden answered on 16 Mar 2016:

We can look at galaxies, and by counting the number of stars, we can estimate how much mass the galaxy has, and so how much gravity there is. Galaxies rotate, and the quicker they rotate, the more gravity is needed to stop them from flying apart. Galaxies all seem to be rotating too quickly, and should be flying apart. The solution to this is that there must be more matter in the galaxies than what we can see. Hence the name dark matter.

So what actually is it? We don’t know. We’ve not known that dark matter exists for very long, and we’re still trying to figure out what it is. The best theory we have at the moment is Weakly Interacting Massive Particles (WIMPs). These are particles that on the whole, do not interact with regular matter and fill the galaxy. In fact they’re passing through you right now. The only way we know that they are there at the moment, is due to its gravitational effects.