Dark matter, the clumps of invisible mass between the stars, may be an illusion - if you tweak the laws of physics slightly, an international meeting of astronomers heard today.

After decades of searching for these elusive intergalactic phantoms, thought to account for 80% of all matter in the universe, there are still problems with theories that describe it and some of these conflict with observations, the 25th General Assembly of the International Astronomical Union in Sydney was told.

"The existence of missing mass in the universe is undeniable - if you think Newton's fundamental laws apply," said Dr Jerry Sellwood, an astrophysicist at Rutgers Universityin New Jersey, USA. But if Isaac Newton was slightly mistaken when he formulated laws governing gravitation and inertia in 1687, dark matter would disappear altogether, he said.

If at very weak accelerations, objects do not slow down quite as rapidly as Newton's inverse square law of gravity predicts, then a lot of the data suggesting dark matter exists could be dismissed, Sellwood told ABC Science Online: "A tiny change like that would allow us to explain what we see, without needing dark matter. And it would not be inconsistent with any experiment to date."

Alternatively, a small change in the law of inertia would do it. "If the acceleration of a particle in response to applied force - when you get down to extremely small forces - experiences a slightly larger acceleration than you would have expected, then that does it too," he said.

Either change would not upset our understanding of physics, but would explain away observations that suggest the existence of dark matter - although Sellwood agreed that changing Newton's laws was "a radical suggestion".

For decades, astronomers have known that there was more matter in the universe than could be seen with telescopes. Clusters of galaxies - and even star and gases within galaxies - have been observed behaving as if they were under the gravitational pull of large objects nearby objects ... objects which do not show up on telescopes.

Whatever dark matter is - and it may be several things - it does not shine brightly, either in visible light, X-rays or at any other wavelengths. Astronomers have detected what appears to be one type of dark matter in a kind of halo around our own galaxy - large chunks of loose interstellar rubble known as MACHOS, or massive compact halo objects.

But there are problems with theories that try to explain dark matter, Sellwood said. If dark matter exists, it should be distributed randomly around a galaxy or a cluster of galaxies: its amount should not be dependent on the amount of visible matter present. But in all cases where it has been reliably detected, dark matter seems to closely shadow its visible companions.

"Light distribution seems to be a very good predictor of the mass of a galaxy, which is not what you expect to see," said Sellwood. "The [total] mass that you get is a much larger number than you would have guessed from visible light - but the extra mass is a pretty much related to the amount that's visible.

"This is very puzzling," he added. "I think it's very difficult to explain this in dark mater models, but it may not be impossible. There is no known explanation … it may require a much deeper understanding than we have of the way galaxies form."

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