Chandra X-ray Image of MACS J0025.4-1222
This image shows a system where two massive galaxy clusters have collided and in doing so have forced the separation between dark and "normal" matter. X-rays from Chandra (pink) show where most of the normal, or baryonic, matter in the cluster resides.
(Credit: X-ray(NASA/CXC/Stanford/S.Allen); Optical/Lensing(NASA/STScI/UC Santa Barbara/M.Bradac))

Hubble Optical and Lensing Images of MACS J0025.4-1222
Optical data from Hubble is used to trace the mass. This result helps answer a crucial question about whether dark matter interacts with itself in ways other than via gravitational forces. It also shows that the Bullet Cluster, a similar system, was not an exception and that the earlier results were not the product of some unknown error.
(Credit: NASA/STScI/UC Santa Barbara/M.Bradac)