Dark matter: a hypothetical form of invisible matter that serves to explain the gravitational conditions in the universe.

The first indication of a hitherto-unknown form of matter goes back to the 1930s, when astronomers were measuring the velocities of the circulation of stars. These were found to orbit at a higher speed than predicted around the centers of their respective galaxies. This was a first indication that galaxies are clearly heavier than the combined weight of their stars together with their dust and gas clouds.

These results were later confirmed by more detailed analyses of the gravitational fields of galaxies and galaxy clusters. Two possible explanations emerged: Either Newton's law of gravitation is not valid at great distances -- according to physicists an unlikely assumption. Or there are gigantic quantities of heavy matter somewhere between the stars -- quantities of a kind of matter that is entirely invisible, does not emit, reflect or absorb any light, and possesses a mass five to ten times that of regular matter.

But of what would this dark matter consist? Possible candidates are dark celestial bodies and as-yet-unidentified elementary particles. String theory postulates the existence of a hypothetical particle, the so-called photino, as the partner of the photon (light particle). We may be able to generate and thus confirm the existence of photinos by means of the new particle accelerator LHC in Geneva, which is scheduled to commence operations in 2009. Another possible candidate is the axion, a hypothetical lightweight particle that is postulated by one of the theories about CP violation.

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