Question: what is dark matter?

Mark Kennedy answered on 14 Nov 2016:

Oh man, what an excellent question. Quite honestly, we don’t know what dark matter is! The person who discovers it is going to get the Nobel Prize in Physics though.

We know that it exists because of experiments and observations that have shown that something has to be inside of galaxies that gives them the structure we see. In fact, Vera Rubin and Kent Ford are credited with discovering it (http://www.astronomy.com/news/2016/10/vera-rubin). In their observations, they saw that the Andromeda Galaxy was moving totally wrong when compared to what they predicted, and that some mysterious matter that they couldn’t see was responsible. They called it dark matter.

Now, we have a few candidates for what dark matter could possible be. But, (as you could guess by the name) dark matter doesn’t reflect light – which means we can’t see it using normal telescopes! Which makes figuring out exactly what it is very hard.