ASTRONOMERS believe they have found the mysterious 'dark matter' that makes up more than 95 per cent of the universe but which has never been seen.
Research published today indicates that some dark matter takes the form of brown dwarfs, planet-sized objects in space that lack the glow of stars and are invisible from Earth.
Two teams of astronomers describe in the scientific journal, Nature, how they detected signs of brown dwarfs using a principle proposed by Albert Einstein nearly 80 years ago.
They monitored millions of stars in the Large Megellanic Cloud the nearest galaxy to our Milky Way and found three that became gradually brighter for a couple of months before dimming again.
They believe this was the result of a brown dwarf passing between the star and Earth and acting as a 'gravitational lens' that magnified and focused the starlight in the same way that a magnifying glass passing over a book momentarily enlarges words on a page.
Einstein theorised in 1916 that light would be magnified, under the influence of gravity. For the past few years, astronomers have been searching for evidence of gravitational lensing caused by large, dark objects such as brown dwarfs.
A US-Australian team, led by Charles Alcock of the Lawrence Livermore National Laboratory in California, and French astronomers, led by Michael Spiro of the Centre d'Etudes de Saclay near Paris, agreed to announce their findings simultaneously.
Christopher Stubbs, an astrophysicist at the University of California at Santa Barbara, said automatic instruments for monitoring stars made the research possible.
At least two other groups are believed to have found similar evidence for the existence of brown dwarfs.
Confirmation of brown dwarfs supports the theory that the mysterious dark matter of the universe is matter known as machos (massive compact halo objects).
Scientists are divided as to whether the dark matter in the universe is in the form of machos, such as brown dwarfs, or exotic sub-atomic particles that are difficult to detect, called wimps (weakly interacting massive particles).
Although the discovery of brown dwarfs supports the 'macho' hypothesis, not everyone is convinced.
Professor Michael Rowan-Robinson at Imperial College, London said that even if the astronomers have finally found brown dwarfs, they cannot account for all the dark matter.
'These new results do not eliminate the case for wimps. It would be premature to write them off,' he said.