>> I was going to put this in my geek thread but since it is really about dark matter, well - here is a geek rap about looking for dark matter.
That made me cringe, but like a car crash, I couldn't stop watching it.
It is like many scientific discoveries; its affects can be seen so it is added in to balance things out. Observations of Uranus showed that something not seen was affecting its orbit. They finally found Neptune. Though, truthfully, it was first seen by Galileo but he is not credited because he thought it was a fixed star. The Neutrino was not seen but physicists needed something to balance certain equations, later they found it.
Well, not to beat a dead horse but now there is Dark Flow.
Using data from NASA's Wilkinson Microwave Anisotropy Probe (WMAP), scientists have identified an unexpected motion in distant galaxy clusters. The cause, they suggest, is the gravitational attraction of matter that lies beyond the observable universe.