Astronomers were puzzled earlier this year when NASA's Hubble Space Telescope spotted an overabundance of dark matter in the heart of the merging galaxy cluster Abell 520. This observation was surprising because dark matter and galaxies should be anchored together, even during a collision between galaxy clusters.
Using the new camera, Clowe's team measured less shear in the cluster's core than was previously found. In the study the ratio of dark matter to normal matter, in the form of stars and gas, is 2.5 to 1, which is what astronomers expected. The earlier WFPC2 observation, however, showed a 6-to-1 ratio of dark matter to normal matter, which challenged theories of how dark matter behaves.
For those interested, this post of about five years ago is of relevance.
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Abell 520 'Dark Core' May Not Be So Darkabell 520
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