bullet cluster

In between working on the first paper for astrometry.net there was a lot of discussion here at NYU about the offset between the mass center and the baryon center of the subclump in the bullet cluster, which got a lot of press because it says some things about dark matter. This is a very hard observation for modified gravity theories to reproduce (at least the ones that want to replace dark matter with a modification to GR). More interesting to me is that the sub-clump is moving incredibly fast (hence bullet), although the theorists say this is not incredibly unlikely.