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There鈥檚 a flood of gamma rays coming at us from the centre of the Milky Way, and dark matter smashing apart at the edge of black holes could be partially responsible.

Supermassive black holes are believed to sit in the centre of every galaxy, and their smaller brethren, intermediate-mass black holes (IMBHs), are probably in the neighbourhood too.

The observed gamma ray excess from that region can鈥檛 be entirely accounted for by ordinary matter, so physicists turn to weakly interacting massive particles (WIMPs) 鈥� a leading contender for the particles that make up dark matter. But recent analyses of fluctuations in those gamma-ray signals indicate something else must be contributing, too. WIMPs annihilating when they meet other WIMPs can explain the overall shape of the gamma-ray emission we鈥檝e detected, but bumps in the signal could only be caused by other sources, such as black holes or pulsars.

at the University of Montpellier in France and at the Sorbonne University in Paris, now propose that these fluctuations could come from a concentration of medium-sized black holes embedded within the central bulge of our galaxy.

Lacroix and Silk calculate that dark matter piling up at the edge of these holes, in what they call dark matter density spikes, should produce enough gamma rays to account for the excess 鈥� so long as there are about one thousand IMBHs in the innermost regions of the galaxy.

Caught in the act

These medium-sized black holes range from tens to hundreds of thousands of solar masses. They are believed to form in the early universe in one of two ways: when massive first-generation stars went supernova and collapsed in on themselves, or growing from the seed of a primordial black hole.

Some theories say these medium-sized black holes could merge to eventually create supermassive holes, but others may get ejected from major mergers and left free to roam in galactic stellar halos.

鈥淭he presence of intermediate-mass black holes in the centres of galaxies isn鈥檛 something many people talk about these days, even though they鈥檝e been predicted,鈥� says Jillian Bellovary at听, New York City, and the . 鈥淲e haven鈥檛 thought a lot about how dark matter might accumulate in clumps (aka density spikes) near IMBHs.鈥�听

Astronomers have struggled to find observational evidence for intermediate-mass black holes until the Laser Interferometer Gravitational-Wave Observatory (LIGO) first detected a merging pair in September 2015. Since then, LIGO has detected another three pairs, and astronomers are re-examining the role these medium-sized monsters might play in galaxy evolution.

If LIGO finds some of these mergers occurring in the centre of other galaxies, it could show that dark matter spikes near medium-sized black holes are indeed behind the gamma-ray excess in our own galaxy.

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