Dark Matter Clouds
A Portrait of Dark Matter
Oops! Please forgive the text showing through from the back of the scanned magazine page.
You cannot see it directly since dark matter does not emit, absorb, or reflect light, but it is there lurking within Pandora's cluster, a gargantuan smash-up of four smaller clusters of galaxies. This composite image, released last summer, captures both the light and dark parts of the cluster. Amazingly, the visible galaxies (white spots) represent only 5 percent of the cluster's mass. Twenty percent consists of diffuse, searing-hot gas (red), as detected by NASA's Chandra's X-ray observatory. The remaining 75 percent is dark matter (blue). So how can scientists "see" the dark matter if it is invisible? Its gravity gives it away. Drawing on data from observatories, researchers analyzed the gravitational distortion of light passing through the cluster and used that information to map the cluster's mass. In studying how the dark matter is distributed relative to the galaxies and gas clouds, they confirmed one of its key characteristics: It does not respond to the electromagnetic forces that affect ordinary matter.
Gillian Conahan--Discover Magazine--November 2011