Astronomers are convinced-there is a type of matter that doesn't interact with electromagnetism but interacts gravitationally. The result? Dark matter-something that we can't see because light is electromagnetic radiation. But we see its effects true enough. If you examine the behavior of galaxies there is clearly mass there that cannot be detected visually (or we don't understand gravitation on large scales).
Personally I find either possibility exciting. But dark matter appears to be more likely. And recently, astronomers are claiming to have detected it. In an analysis of the collision of two galaxy clusters, the matter, which consisted of hot visible gas-cooled and slowed down. Just like you would expect. But the "dark matter" did not. This indicates that aside from gravitation it interacts weakly if at all. This is experimental support for the existence of dark matter, something that has only been inferred so far.