What is Dark Matter?
Short answer: nobody really knows. In fact, it may not be matter, or it may not exist. As of now, Dark Matter is really just a hypothetical construct that allows the formulas that describe gravity to work.
Longer answer: If you add up all the stuff in the universe – all the planets, stars, asteroids, black holes, comets, nebulae, dark clouds, etc. – it’s not enough matter to account for all the gravity operating in the universe. All the visible matter is only about 15% of what’s necessary to account for all the gravity.
Thus, a good description of Dark Matter is the yet to be verified/detected “stuff” that provides 85% of the gravity. Maybe its a bunch of brown dwarf stars or super massive black holes. Maybe its undetectable matter of a sort that we haven’t discovered yet – there are all sorts of theories about this different type of matter. Or another possibility is that the laws of gravity as we know them are wrong.
The hypothetical existence of dark matter has been known since the 1930s and verifying its existence is the longest standing, unsolved problem in modern physics.
A future physics Friday IFOD will be about Dark Matter.