Astro Abell...
The Chandra image shows a smooth increase in the intensity of X-rays all the way into the central galaxy of the cluster. These X-rays are produced by the multimillion degree gas, which is confined to the cluster primarily by the gravity of the dark matter. By precisely measuring the temperature and intensity distribution of the X-rays, astronomers were able to make the best map yet of the distribution of dark matter in the inner region of the galaxy cluster.
The X-ray data imply that the density of dark matter increases smoothly all the way into the central galaxy of the cluster. This discovery agrees with the predictions of cold dark matter models, and is contrary to other dark matter models that predict a leveling off of the amount of dark matter in the center of the cluster.
If Abell 2029 is a representative sample of the universe, these results indicate that 70 to 90 percent of the mass of the universe consists of cold dark matter
The most detailed plot of dark matter at the heart of a galaxy cluster suggests that about three-quarters of the cluster's mass and the mass of the universe exists as "cold" dark matter subatomic particles that were moving slowly when the first galaxies were born.
Theories involving cold dark matter predict that the density of dark matter in a cluster will increase steadily toward its center, while other theories predict the density will decrease. Chandra's high-resolution observations showed a gradual rise in the density into Abell 2029's central galaxy.
A similar result was reached in an earlier study of the less orderly Hydra A galaxy cluster. Its dark-matter density also appeared to increase toward the cluster's center.
But now both investigations seem to suggest that cold dark matter composes the bulk of the universe.
Cold dark matter gets its name from the assumption that cold dark matter particles were moving slowly when galaxies and galaxy clusters began to form. The exact nature of these particles is still unknown.
The new Chandra observations include estimates of the total energy content of the Universe. As shown in this illustration, dark energy is estimated to contribute about 75% of the energy in the Universe, dark matter about 21% and normal matter about 4%. Only the normal matter can be directly detected with telescopes, and about 85% of this is hot, intergalactic gas, as detected in Chandra observations of galaxy clusters.
The red diffuse emission shows hot intergalactic gas, heated to about 100 million degrees by the enormous gravity in the cluster, and visible only in X-rays. The distances to the clusters in the sample can be derived from the Chandra observations by calculating the relative amounts of hot gas and dark matter. These distances show that the expansion of the Universe began accelerating about six billion years ago.
Reference:
http://chandra.harvard.edu/photo/2003/abell2029/
http://www.astronomy.com/Content/Dynamic/Articles/000/000/001/378ynaxn.asp
http://www1.msfc.nasa.gov/NEWSROOM/news/photos/2004/2004images/pie_m.jpg
