Hints that dark matter could be crashing and burning in the centre of the Milky Way might themselves be going up in smoke. A search for nearby dwarf galaxies that should show the same signal have proved fruitless, leaving physicists disappointed after five years of excitement.

The signal first emerged in 2010, when Dan Hooper of the Fermi National Accelerator Laboratory and colleagues found an unexpectedly bright gamma ray glow in data from the Fermi Gamma-Ray Space Telescope. They interpreted it as the debris left behind when particles of dark matter – the mysterious substance that makes up most of the matter in the universe yet refuses to interact with ordinary matter except through gravity – crashed together and annihilated each other in the centre of the Milky Way.

At the time, nobody could explain the glow in less exotic terms. And what’s more, the theory fitted well with what physicists expected might happen if dark matter was made up of relatively lightweight particles, with a mass of about 10 to 50 gigaelectronvolts (GeV).


In 2013, Hooper and Tracy Slatyer at the Massachusetts Institute of Technology boosted the case further by showing similar signals just outside the galaxy’s core.

The case seemed stronger still last year when Kevork Abazajian at the University of California, Irvine, and colleagues found signs that the remnants of annihilated dark matter particles were scattering off dust in the Milky Way, just as physicists predicted they should. All the evidence seemed to support the idea that the extra gamma rays were caused by dark matter.

Signal failure

But this year, the theory started to unravel. In July, two independent teams found thousands of previously unnoticed pulsars – corpses of dead stars – that could account for the extra gamma rays.

There was one last test that dark matter could make it through. The Milky Way isn’t the only galaxy that ought to have dark matter being crushed together at its centre. If dwarf galaxies have the same signal, then the dark matter interpretation could still hold water.

But when Abazajian and his student Ryan Keeley analysed data from nearby dwarf galaxies released by the Fermi telescope this year, they found no such signal there. Abazajian says this rules out the dark matter interpretation at a roughly five sigma level, which physicists take as the benchmark for statistical significance.

“If the dwarf limits were this strong in 2010-12 when we first started working on the dark matter interpretation of the galactic centre excess, we may have not taken it as seriously,” he says.

Katherine Mack of the University of Melbourne, Australia, is also losing hope. “Bringing this whole picture together, it’s looking less and less likely that you can bring together the idea that the galactic centre excess is dark matter annihilation,” she says.

But not everyone is so sure yet. Slatyer says there are still models that would fit this data. She says a different type of work will be needed to prove the gamma rays aren’t coming from dark matter. “I think the best way to conclusively falsify a dark matter interpretation of the excess would be to demonstrate that the emission is coming from a different source,” she says.

Hooper also remains optimistic: “There is a little tension here in my option, but not enough to significantly move my opinion on the subject one way or the other.”

Regardless, Mack says the whole exercise has been far from a waste of time. “We’re really exploring the galactic centre in a way that we might not have been able to do if we hadn’t been checking out these signals looking for dark matter,” she says. “We’re learning a lot about high-energy astrophysics.”

Journal reference: arxiv.org/abs/1510.06424

Image credit: NASA/DOE/Fermi LAT/D. Finkbeiner et al.