Dark Matter, apparently not modern-day epicycles after all

Well, John Baez was right, the mysterious NASA press release about a dark matter discovery was about the Bullet Cluster.

The discovery provides clear evidence in favor of dark matter, and against MOND. In the Bullet Cluster, when two clusters of galaxies collided, the regular matter in each cluster interacted, causing it to slow down, while the dark matter just kept going on its merry way. So while in a normal galaxy the gravitational forces pull towards the center of the visible galaxy, in the Bullet Cluster, the pull is now towards the bulge of dark matter to the side. It would be difficult to explain the discovery in the context of MOND — you would have to explain why in these particular galaxies the gravitational pull is asymmetric, while in most galaxies it’s symmetric.

So years of internet pundits are apparently wrong. Dark matter is not the new Ptolemaic epicycle. NASA’s article is here. Some animations of the collision can be found here.