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It's dark out there...

"Dark matter" refers to matter of an unknown nature that many astronomers and cosmologists think must make up the majority of the mass in the universe. Its presence is revealed by the gravitational effects on objects that we can see. According to the current understanding of how gravity works, the way the visible matter behaves indicates that there should be much more matter than we can detect — and therefore, much more mass exerting a gravitational influence — in objects in space, like stars in galaxies, or galaxies in clusters. The clusters move at speeds that are too high to be attributed just to the visible galaxies.

If we were to apply the rules of gravity to the matter that we can see, galaxies (and galaxy clusters) would fly apart, losing the swiftly moving outer components, because there isn't enough mass (and therefore gravity) present to hold them in place. So, if more mass is added to the visible matter, the equations work, objects remain in their paths, and everything makes sense, mathematically. Results from the Wilkinson Microwave Anisotropy Probe (WMAP) show that roughy 1/4 of the mass of the universe is composed of dark matter. But what is it, really?

There are many theories as to what comprises dark matter. It is unlikely to be any one substance, but rather a variety of substances that contribute to the total needed to hold things together. Some aspects are fairly certain: the objects would not give off much, if any, visible light, and so may include black holes, brown dwarfs, neutron stars, red dwarfs, and planets;  they may also be very small individually, distributed somewhat equally in a very large volume, like a cloud. That could include particles such as axions, neutrinos and neutralinos, as well as other particles, both exotic and commonplace. However, there are still many missing pieces to this puzzle, and scientists continue to search for more pieces.

While the individual components are difficult to find, the influence of dark matter is easily detected, and comparatively simple to measure. 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 mass, hidden from view. 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 several times that in the "visible" galaxies alone. Additionally, a phenomenon called gravitational lensing acts as a more visually obvious way to demonstrate how a galaxy's mass (and therefore gravity) can bend the rays of light traveling from a distant object to Earth.

For the time being, dark matter can be thought of as a way to fill in a variable in an equation — the definite quantity of indefinite substance, or the indefinite influence of some as-yet undetectable force. As some scientists explore the possibilities of known-substance candidates for dark matter, others are also searching for new "supersymmetric" particles, i.e., hypothesized particles that are partners to the particles that are already known, as with the research being done by the European Organization for Nuclear Research, or CERN (, with the LargeHadron Collider ( Also, some cosmologists hypothesize that gravity from our dimension could be exerting influence on matter in other dimensions, and perhaps vice-versa, which is also among the LHC research objectives.

Last Updated: February 2011

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

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