From Wikipedia:
The dark matter component has vastly more mass than the "visible" component of the universe. Only about 4% of the total energy density in the universe can be seen directly. About 22% is thought to be composed of dark matter. The remaining 74% is thought to consist of dark energy, an even stranger component, distributed diffusely in space. Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics. It has been noted that the names "dark matter" and "dark energy" serve mainly as expressions of our ignorance, much as the marking of early maps with "terra incognita".
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Dark Matter copyright Daniel Rowe, all rights reserved (posted 27 Aug 2007, 10:28)