I think you're confusing anti matter with dark matter.

But for dark matter you're pretty much right. For example, we can see things orbiting with some speed and can deduce the mass needed for such an orbit. The mass of stuff we see is mich smaller than the needed mass, so apparently there is some stuff that is "dark", i.e. doesn't shine or reflect light, also doesn't absorb. More like glass in this respect, or air. The light just doesn't care about it being present. So physicists started calling it "dark matter" as it doesn't shine, plus it fits well with the fact that we don't know yet what it is.