Bullet Cluster

The Bullet Cluster is a specific pair of galaxy clusters about 1.141 Gpc distant from us, which appear to have collided with each other. The name actually refers to the smaller of the two. The intracluster medium (ICM) (plasma) looks as if it were combined by the apparent collision, but surrounding gravitational lensing is consistent with dark matter that passed through as though unaffected by electromagnetic effects. This is inconsistent with the alternatives to dark matter that have been theorized, such as MOND, which explains the dynamics of galaxies and galaxy clusters by positing additional characteristics of gravity over long distances rather than presuming some hidden mass.