Using the Chandra X-Ray Observatory, among others, astronomers studied a galaxy cluster formed by the collision of two galaxies. By mapping the movement of the hot gas in those galaxies, they discovered that the gravitational influences were not in the center of the gas cloud, but rather seemed to have been influenced by some outside form of unseen matter - exactly as predicted by dark matter theories.
This was the clearest example ever observed of dark matter separating itself from ordinary matter. Dark matter, which is theorized to be 6 times more abundant than "ordinary matter," interacts only through gravitational forces and exhibits no electromagnetic force, unlike the gas within the galaxy cluster.
The astronomers that have spoken up on the issue thus far seem convinced that this has proven the existence of dark matter, though as with most things in the world, I am sure the distractors will speak up soon enough to present their counter arguments.
- Draft version of the academic paper "A Direct Empirical Proof of the Existence of Dark Matter" in pdf format.
- Cosmic Variance blog - Dark Matter Exists (a great explanation)
- Science - Dark Matter Exposed?
- New Scientist Space - Cosmic smash-up provides proof of dark matter
- Nature - Dark matter spied in galactic collision
- Scientific American - Colliding Clusters Shed Light on Dark Matter
- Yahoo! News/Reuters - "Dark matter" is real
- Chandra Photo Album - NASA Finds Proof of Dark matter, including an animation of the collision
- NASA Finds Direct Proof of Dark Matter
- NASA's Chandra X-Ray Observatory