Most of the matter in the universe gives out no light, or at least so little that it is currently undetectable. Yet we know it is out there because its gravity keeps stars and galaxies in their orbits. Pretty much everyone thinks that this so-called dark matter is made of hitherto undiscovered subatomic particles. Physicists are hopeful they will find a candidate in high-speed collisions at the Large Hadron Collider at CERN, near Geneva, Switzerland.
But could we have got dark matter all wrong? Mike Hawkins thinks so. He believes that rather than particles, what we call dark matter is actually legions of black holes created shortly after the big bang.
It is a controversial claim. Yet Hawkins, who is an astronomer at the Royal Observatory, Edinburgh, in the UK, believes he has persuasive evidence gathered over years of observations. If he is right, it radically changes our picture of the cosmos. "We live in a universe dominated by black holes," he says.
The story dates back to 1975, when Hawkins began monitoring a portion of the southern sky, night after night, using the UK Schmidt Telescope at Siding Springs in Australia. After five years, in addition to the stars he was looking for, he found something unexpected: thousands of objects that were brightening and dimming extremely slowly. They turned out to be quasars, the super-bright cores of newborn galaxies at the edge of the universe.
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