X-ray, Optical & Lensing Images of Abell 520
Astronomers have discovered a chaotic scene unlike any witnessed before in a cosmic "train wreck" between giant galaxy clusters. NASA's Chandra X-ray Observatory and optical telescopes revealed a dark matter core that was mostly devoid of galaxies, which may pose problems for current theories of dark matter behavior.
(Credit: X-ray: NASA/CXC/UVic./A.Mahdavi et al.; Optical/Lensing: CFHT/UVic./A.Mahdavi et al.)

Illustration of Abell 520 System
An artist's illustration of the Abell 520 system shows where the bulk of the matter (blue) is found compared to the individual galaxies (yellow) and the hot gas (red) in the aftermath of a massive galaxy cluster collision. The material shown in blue is dominated by dark matter. As with the Bullet Cluster there are large separation between the regions where the galaxies are most common (peaks 2 and 4) and where most of the hot gas lies (peak 3). However, unlike the Bullet Cluster, a concentration of dark matter is found (peak 3) near the bulk of the hot gas, where very few galaxies are located. In addition, there is an area (peak 5) where there are several galaxies but very little dark matter. These observations conflict with the general understanding that dark matter and the galaxies should remain together, despite a violent collision. This raises questions about the current understanding of how dark matter behaves.
(Illustration: NASA/CXC/M.Weiss)

Comparison of Abell 520 and Bullet Cluster
While there are certain similarities between Abell 520 and the so-called Bullet Cluster, other significant differences raise interesting scientific questions. For example, the dark matter and the galaxies in the Bullet Cluster remained together despite the enormous collision between the galaxy clusters. In the case of Abell 520, these components have been separated, which flies in the face of conventional understanding of dark matter.
(Illustration: NASA/CXC/M.Weiss)