The mystery of dark matter

January 08, 1993

Scientists have long suspected that there is more to the universe than meets the eye. Now astronomers have found the most convincing evidence yet that large amounts of previously undiscovered matter are floating around the cosmos -- even though they still can't see it.

The evidence comes from the X-ray part of the spectrum studied by a satellite which found a huge cloud of hot gas with more mass than 500 billion suns amid a trio of obscure galaxies known as NGC2300.

The hot gas would have boiled away long ago unless a powerful gravitational source were holding it together, from which scientists infer that an enormous -- but invisible -- concentration of matter must lie in the cluster.

From the size of the cloud, which has a diameter of about 1.3 million light years, astronomers calculate that the matter holding it together must be 12 to 25 times the amount contained in the visible galaxies. Thus whatever is there must be some form of the elusive "dark matter" predicted by theory but never observed directly.

The discovery lends support to the argument the universe holds enough matter to eventually halt the cosmic expansion, in which all the galaxies appear to be flying away from each other.

If galactic clusters are found elsewhere in the universe with comparable amounts of dark matter, their gravity would be sufficient to curve the structure of space back on itself so that, like the surface of a sphere, it would be finite yet unbounded. In such a universe one could travel indefinitely in any direction without ever coming to the "edge" of space or time.

There might even be other, more exotic explanations. In the 1960s, astronomer Fred Hoyle speculated that advanced forms of life could take the form of giant interstellar clouds held together by powerful magnetic forces. In his fanciful scenario, such creatures periodically recharged their energy by absorbing vast amounts of stellar radiation -- collected during just the sort of celestial pit stop the cloud in NGC 2300 appears to be making.

Scientists will want to scrutinize this cloud with a wider array of instruments. Whether the cloud's significance lies in serving as a marker for dark matter, or whether it ultimately points to possibilities undreamed of outside the stuff of science fiction, it is a discovery of the first magnitude.

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