X-rays, dark matter and galaxies in cluster Abell 2744
This image combines visible light exposures of galaxy cluster Abell 2744 taken by the NASA/ESA Hubble Space Telescope and the European Southern Observatory's Very Large Telescope, with X-ray data from NASA's Chandra X-ray Observatory and a mathematical reconstruction of the location of dark matter.
The galaxies in the cluster, while they are the only part that is visible in the optical part of the spectrum, actually only provide around 5% of the mass in the cluster.
Hot intracluster gas (shown in pink) is visible through its X-ray emissions, observed by NASA's Chandra satellite.
The blue overlay shows a map of the mass in the cluster. This is reconstructed based on detailed analysis of the way that the cluster bends light from galaxies in the distant background. Evidence of this light bending can be seen in arc-like distortions in parts of this image. Since dark matter makes up the lion's share of mass in the cluster - around 75% - this blue overlay reveals the location of the otherwise invisible dark matter.
Analysis of this data has allowed scientists to observe some strange phenomena in Abell 2744, including a pocket of dark matter with no gas or galaxies, and a clump of galaxies with no associated gas. Astronomers believe that Abell 2744 formed from the simultaneous pile-up of at least four separate clusters.