Physicists are reporting that they’ve found direct evidence of the existence of “dark matter,” the missing mass whose existence was merely inferred based on the observed characteristics of the universe. In the image there, you can see two galaxy clusters. Overlaid on the image are two colored blobs: the blue blob shows the center of mass of the clusters as inferred from gravitational lensing effects. The red blob shows the location of the bulk of ordinary matter, which is in the form of hot gasses. Scientists hypothesize that the clusters passed through each other, and the hot gasses in each cluster collided. As a result, most of the ordinary matter was dragged behind the dark matter in those clusters, giving us the opportunity to observe them separately. You can see an MPEG animation of how scientists hypothesize the collision went.
I don’t know enough about astronomy to venture an opinion about whether this analysis is right, but it’s pretty cool if it is.