If our best sign yet of dark matter is what it seems, then the supermassive black hole at the centre of our galaxy is a complex beast.

Dark matter is thought to make up 80 per cent of the universe’s matter, but it is hard to detect as it scarcely interacts with other matter. One way to sense it indirectly is through high-energy photons emitted when two dark matter particles collide and annihilate each other.

NASA’s Fermi space telescope has seen signs of such photons around the supermassive black hole at the centre of the Milky Way, where dark matter is expected to cluster. Some think it is our best sign of dark matter so far.

But now Jessie Shelton at the University of Illinois at Urbana-Champaign and her colleagues argue that if the signal is truly from dark matter, it is an order of magnitude too weak to match up with conventional ideas of how the black hole formed.


“If the simplest model of black hole formation is the true history of our galaxy, then no reasonable dark matter candidate can be responsible for generating the Fermi excess,” she says.

Exotic growth

The simplest model says our black hole formed gradually from a single seed, slowly eating up the stars and smaller black holes around it. That gentle evolution would compress surrounding dark matter into a dense spike where particles would annihilate more often and yield a stronger photon signal.

But if the black hole grew more exotically – by cannibalising supermassive black holes from neighbouring galaxies that crashed into the Milky Way, say, or forming from many seeds – the dark matter around it would move faster and therefore be more spread out, leading to a weak signal like the one Fermi sees.

Another option is tweaking our theories of how dark matter behaves, but that’s difficult to do, Shelton says. One possibility is that dark matter particles interact with each other via a mysterious “dark force”, bringing them together to annihilate more often over the history of the universe. That would mean there is less dark matter annihilating around the black hole today, reducing the signal.

But if dark matter self-annihilated so easily, then we should have already seen it doing so elsewhere in the galaxy, Shelton says.

Of course, this reasoning depends on the Fermi signal being caused by dark matter.

“I think this calculation is a very interesting and important thing to do,” says Katherine Mack at the University of Melbourne, Australia. “But I don’t know that the Fermi signal is going to hold up as dark matter. There have been lots of promising signals that have turned out to be something else.”

Journal reference: Physical Review Letters, doi.org/wfn