What is Dark Matter?

Question

I was asked this question and had no answer. I would like if someone could please answer.

Answer 1

It is a hypothetical form of matter that interacts very weakly with regular matter that is presumed to fill the universe to account for the much greater galaxy mass (as inferred by their gravity) than can be accounted for by regular matter. Dark matter particles have never been individually observed, but there are a few candidates corresponding to particles allowed for in the Standard Model.

Answer 2

Dark matter is of unknown composition that does not emit or reflect enough electromagnetic radiation to be observed.