The existence and nature of dark matter have stumped physicists for a while now. It cannot be seen (hence the "dark"); it's existence can only be inferred through the effect its gravity has on the stuff we can see. By that measure, there may be as much as six times the amount of dark matter in the universe as there is ordinary matter. Of course, that predominance would make dark matter the ordinary matter, but let's not get into that.
A scientific paper to be published in about two weeks may finally present evidence that dark matter has been detected. Speculation has it that the key is annihilation. When two dark matter particles come into contact, they annihilate each other in an explosion that produces electrons and positrons. Electrons are everywhere, but positrons are not. So, if we detect an over abundance of positrons, we might have evidence of dark matter. The upcoming paper might report just such a detection.