Mapping the Dark Matter in the Core of Galaxy Cluster Abell 520 (image) Ohio University Share Print E-Mail Caption These composite images taken by two different teams using the Hubble Space Telescope show different results concerning the amount of dark matter in the core of the merging galaxy cluster Abell 520. Dark matter is an invisible form of matter that astronomers believe is the underlying gravitational "glue" that holds galaxies together. In the top image observations of the cluster, taken by D. Clowe with the Advanced Camera for Surveys mapped the amount of dark matter in Abell 520. It reveals an amount of dark matter astronomers expect based on the number of galaxies in the core. The dark-matter densities are marked in blue, and the dotted circle marks the dark-matter core. The map is superimposed onto visible-light images of the cluster. In the bottom image a second team, led by James Jee, used the Wide Field Planetary Camera 2 to find an unusual overabundance of dark matter in the cluster's core, denoted by the bright blue color at image center. The observation was surprising because astronomers expect that dark matter and galaxies should be anchored together, even during a collision between galaxy clusters. This discrepancy between the two results requires further observation and analysis, say researchers. The two dark-matter maps were made by detecting how light from distant objects is distorted by the galaxy clusters, an effect called gravitational lensing. Abell 520 is located 2.4 billion light-years away. Credit Credit: (top) D. Clowe, (Ohio University, (bottom) J. Jee (University of California, Davis) Usage Restrictions Credit: (top) D. Clowe, (Ohio University, (bottom) J. Jee (University of California, Davis) Share Print E-Mail Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.