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’s mass, but interacts with regular matter through gravity alone. That means light doesn’t 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’t 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’s outskirts – where there is more dark matter than regular matter – appear brighter than they would otherwise (Physical Review Letters, doi.org/z8s). This would produce a glow at the periphery, where it is easier to observe and doesn’t have to compete with light from the galaxy’s stars, says Davis. This effect might be stronger in lower-energy light, like infrared instead of ultraviolet.


“It’s a clever idea,” says Michael Boylan-Kolchin, an astronomer at the University of Maryland. “Since we haven’t yet detected dark matter directly, it’s important to constrain its properties in as many ways as possible.”

This article appeared in print under the headline “Jostling photons could give dark matter away”