Astronomers shed light on mystery of 'dark matter'

By Oliver Duff

Published: 06 February 2006

British astronomers have come a step closer to understanding one of the most mysterious substances in the universe - the "dark matter" which acts as a kind of cosmic glue, holding galaxies together.

New research into dark matter, believed to make up 23 per cent of the cosmos compared to the 4 per cent of "normal matter" that can be seen and felt, has for the first time pinned down its behaviour and properties, which are essential to anchoring an ordered universe.

Scientists assumed the existence of dark matter in the 1930s after "weighing" galaxies and finding that the sum of each galaxy's parts was much less than its total mass.

They have speculated ever since on what the "missing mass" might be. A team led by Professor Gerry Gilmore from Cambridge University's Institute of Astronomy began the latest experiment three years ago in northern Chile's spectacular Atacama desert - a place with "no birds, no animals, just sad little astronomers", he said.

Using the Very Large Telescope, the real name for an array of interconnected 26ft-wide telescopes, to look at a dozen "dwarf" galaxies that hang on to the coat-tails of our own galaxy, the Milky Way, the astronomers claim to have achieved astonishing results.

"We've got the first clue as to what dark matter is," Professor Gilmore. "Which is that you only find this stuff in big, magical, rather dilute lumps, about 1,000 light years across, 40 million times the mass of the Sun." However, he admitted: "We don't know how to interpret this clue yet."

Dark matter does not give off any light, hence its name. Scientists had always assumed that because it couldn't be seen it was "cold" - a sort of dead, sluggish cosmic sludge. But there were two further unexpected findings from the Cambridge research. The first showed that dark matter actually has a "temperature" higher than that of the surface of the Sun.

If it was made of hydrogen atoms, dark matter would be 10,000C and appear as a blinding light. Yet, confusingly, it does not give off any heat.

The second surprise was that particles of dark matter zip about at 9km per second and are loosely packed.

They are transparent to light, and unlike most particles of ordinary matter, have no electric charge. But they are weighty enough to exert a gravitational pull that prevents the stars in galaxies from flying apart.

A paper on the research is in the final drafting stage and should appear in a scientific journal soon.

It is not time for astrophysicists to relax quite yet, however. Once the question of dark matter is resolved, there is the question of the remaining 73 per cent of the cosmos - made up of something even more mysterious called dark energy, which is forcing galaxies apart at increasing speed.

"It's fair to say there is more work to do," said Professor Gilmore.

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