Here's a new theory that does without Dark Matter:-http://www.physorg.com/news/2011-08-dark-illusion-quantum-vacuum.html-It postulates that antimatter and matter have opposite "gravitational charges" analogous to positive and negative electricity. -I like the idea. It could explain a lot of things. -On the other hand it could be totally wrong of course. -It can be tested, so it's good science.