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It's Dark Out There....

"Dark matter" refers matter of an unknown type that astronomers and cosmologists believe must make up the majority of the mass in the universe. It is called "dark" because it does not emit any light. We know of its presence because of the gravitational effects it has on objects that we can see. For example, galaxies in clusters move at speeds that are too high to be attributed just to the visible galaxies. In addition, astronomers measure high temperature gas in these galaxy clusters. This gas is at too high a temperature to remain bound to the cluster without some additional, hidden, mass. For galaxies and groups, the X-ray data have often indicated very extended dark matter halos far beyond the radius at which one sees starlight or galaxies. The total inferred dark matter mass is often 10 times that in the " visible" galaxies alone.

Dark matter also plays a role in the early universe. Astronomers theorize that the presence of dark matter helps to explain the relative amounts of light elements and isotopes produced in the Big Bang. Results from the Wilkinson Microwave Anisotropy Probe (WMAP) show that 23% of the Universe is made up of dark matter.

One of the best ways of determining the mass of a system, such as a cluster of galaxies, group of galaxies or a massive elliptical galaxy, is to measure the X-ray temperature and gas profiles. Astronomers start by assuming the gas is in equilibrium, which is borne out by the thermal spectra of the gas. Matching models of the distribution and temperature of the gas to the X-ray observations gives the mass of the gas. Use of this technique has shown that clusters of galaxies are gas and baryon rich, that is, the mass in gas exceeds the mass in stars by factors of 3-5 and that the total baryonic mass is ~15% of the total mass of the cluster (i.e. visible mass plus dark matter). Since clusters are supposed to be "fair samples of the universe" they should have a baryon fraction that corresponds to 4%, as inferred from Big Bang nucleosynthesis and results from WMAP. However, WMAP and other evidence now point to a new component in the universe, which is called dark energy. Dark energy makes up 73% of the energy and matter of the universe. By including this dark energy with the visible mass and dark matter, we find that clusters really do share in the same baryonic fraction as the rest of the universe.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Nicholas White (Director), within the Exploration of the Universe Division (EUD) at NASA's Goddard Space Flight Center.

The Imagine Team
Project Leader: Dr. Jim Lochner
Curator:Meredith Bene
Responsible NASA Official:Phil Newman
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