Astronomers observe how the universe moves and deduce, using known laws of physics, how much matter there must be to make it move as observed, since gravity (from matter) influences the motion of every object. There isn't enough visible matter to account for observed motion, so (assuming we understand gravity/relativity well) there must be something we don't see that also adds more gravity. Astronomers don't agree yet on just what it is we don't see, i.e. what kind of matter is out there, but it may be totally different from the stuff that makes up you and me and planet Earth and all the stars. New observations are narrowing the choices, but I have the feeling that professional astronomers argue among themselves a lot over this one.