Galaxy cluster. X-ray image of the galaxy cluster Abell 2029, which lies about one billion light years from Earth. The galaxies and gas are confined to the cluster due to the gravity of its dark matter (particles that emit little light that were left over from the dense early universe). If Abell 2029 is representative of the rest of the universe, this indicates that between 70 and 90 percent of the mass of the universe consists of dark matter.