Abell 520: A system where at least two massive galaxy clusters have collided.
Caption: This multi-wavelength image shows the chaotic aftermath of the collision of at least two galaxy clusters, some of the most massive objects in the Universe. X-rays from Chandra (red) show the hot gas that envelopes the clusters. The individual galaxies appear in visible-light observations (yellow and orange), which also reveal the presence of dark matter (blue) by the subtle distortions of the distant objects. The behavior of the dark matter with respect to the galaxies and hot gas in Abell 520 is very unusual. These data can be explained by changes to the current understanding of dark matter or how galaxy clusters interact when merging.
Scale: Image is 9.25 arcmin across.
Chandra X-ray Observatory ACIS Image