Dark Matter Core Defies Explanation in Hubble Image

This composite image shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520, formed from a violent collision of massive galaxy clusters. The natural-color image of the galaxies was taken with NASA's Hubble Space Telescope and with the Canada-France-Hawaii Telescope in Hawaii.Superimposed on the image are "false-colored" maps showing the concentration of starlight, hot gas, and dark matter in the cluster.