Abell 520 galaxy cluster. Composite image showing the distribution of dark matter, galaxies and hot gas in the core of the merging galaxy cluster Abell 520, which lies around 2.4 billion light years from Earth. Optical data from the Hubble Space Telescope (HST) and the Canada-France-Hawaii Telescope (CFHT) is shown in red, green, and blue. Starlight from galaxies within the clusters, derived from CFHT observations and smoothed to show the location of most of the galaxies, is coloured orange. This reveals that a clump of dark matter resides near most of the hot gas, where very few galaxies are found. Imaged between October 2000 and January 2008.