Steve Connor: The mystery that has endured since Big Bang

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The Independent Online

For anyone who has been mesmerised by the sheer number of stars that make up a clear night sky, it seems incredible that what we can see, even with a telescope, is but a small fraction of what is actually out there. In fact, more than 80 per cent of the material of the universe is invisible to even the best instruments.

It is called "dark" matter because, unlike the "bright" matter of the visible stars, galaxies and planets, it is invisible, even though its gravitational presence can be felt. What dark matter is made of, however remains a mystery.

Fritz Zwicky, a Swiss astronomer, was the first to postulate the existence of dark matter in 1933 when he observed clusters of galaxies beyond our own Milky Way. Zwicky said that these distant galaxies were moving too fast to be held together by the gravity of the visible stars they contain.

Confirmation of Zwicky's idea came in the 1970s when astronomers measured the speed at which stars moved inside and on the outer edges of galactic discs. To their surprise, the outer stars were travelling just as fast as the inner stars. Gravitational theory suggested that the outermost stars should be travelling more slowly.

The only reasonable explanation was that each galaxy had up to 10 times more mass than could be seen. This extra material was creating the additional gravity that kept the outer stars from slowing down.

The latest findings from the Hubble space telescope, released at the American Astronomical Society in Seattle yesterday, suggest that dark matter forms an invisible "scaffold" around which the ordinary matter of the stars and galaxies have formed. The map of dark matter has been likened to a three-dimensional X-ray of the skeleton on which the "flesh" of the visible universe is hung.

Knowing the whereabouts of the dark matter is critical to understanding how galaxies formed and how they began to accumulate into clusters over the 13.7 billion years since the Big Bang.

Critically, comparing the distribution maps of bright and dark matter may point to important differences between them. Several experiments on Earth are designed to capture the elusive subatomic particles that may account for the missing mass.

Many scientists now feel that we are on the verge of discovering what it is that has formed such an immense part of the universe.

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