Actually, dark matter is just a theory.
It's like... a physical manifestation of mankind's ignorance - a big, black, undetectable mass that exerts gravity on things around it. The name itself belies our ignorance - it's just something randomly made to fill up holes in our reasoning and observation. It's about as close to an actual physical manifestation of our ignorance as we're going to get.
I don't put much stock in the theory, it's just used to explain "inconsistencies" in gravity through our limited understandings of how it works. It's how we fill in the gaps of our knowledge. We make up remarkably silly things to explain how we think things work, instead of saying, "We just don't know," or, "This doesn't make sense to us - yet."
I think there's a lot more to be seen and understood about gravity and the universe, and that the idea of "dark matter" is rather silly. Dark matter is quite literally the Phlogiston of today.
