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DARK matter may not be so dark after all. If the invisible stuff interacts with light, it could glow bright enough to be seen.

Dark matter is thought to make up 85 per cent of the universe鈥檚 mass, but interacts with regular matter through gravity alone. That means light doesn鈥檛 radiate from it or reflect off it. But direct searches for dark matter have come up empty, leading some physicists to suspect the theories aren鈥檛 right.

So Jonathan Davis and Joseph Silk at the Astrophysical Institute of Paris, France, asked what would happen if light could interact with dark matter. They calculated the interaction rate needed between photons and dark matter to produce an observable glow.

They found that even if photons collide with dark matter particles a hundred to a million times less often than they would with normal matter, their interaction would still make a spiral galaxy鈥檚 outskirts 鈥� where there is more dark matter than regular matter 鈥� appear brighter than they would otherwise (Physical Review Letters, ). This would produce a glow at the periphery, where it is easier to observe and doesn鈥檛 have to compete with light from the galaxy鈥檚 stars, says Davis. This effect might be stronger in lower-energy light, like infrared instead of ultraviolet.

鈥淚t鈥檚 a clever idea,鈥� says Michael Boylan-Kolchin, an astronomer at the University of Maryland. 鈥淪ince we haven鈥檛 yet detected dark matter directly, it鈥檚 important to constrain its properties in as many ways as possible.鈥�

This article appeared in print under the headline 鈥淛ostling photons could give dark matter away鈥�