What is dark matter?
Many astronomers will tell you that it’s a mysterious substance, as yet unknown to physics. It has many strange properties, including invisibility. And the Universe contains a lot of it: there’s four to five times as much dark matter as there is ordinary matter (the stuff that makes up stars, planets, and so on). In other words, for every galaxy you see, there’s four or five galaxy’s worth of stuff that you don’t see.