In this false-colour image of the centre of the galaxy taken by the Fermi space telescope, all known gamma-ray sources have been removed, revealing excess emissions that may arise from dark matter annihilations (Image: T. Linden, University of Chicago)

Things are looking brighter than ever for dark matter. A brilliant haze of gamma rays coming from the centre of the Milky Way is increasingly likely to be a sign of dark matter particles annihilating each other in space. Meanwhile, hints of the same signal coming from dwarf galaxies now strengthen the case.

“This is the most compelling signal we’ve had for dark matter particles – ever,” says Dan Hooper at the Fermi National Laboratory in Batavia, Illinois.

Hooper and his colleagues have been studying this signal since 2009, steadily building the case that dark matter is the cause. In the latest work, the team say the particle must be heavier than they first thought, bringing it in line with some of the simplest theories of dark matter. But there is a twist: a heavier particle would be in conflict with whiffs of the elusive substance from experiments trying to directly detect the particles.


Gamma ray glow

Dark matter is thought to make up more than 80 per cent of the matter in the universe. So far we have not seen it interact with ordinary matter except via gravity, and no one knows what the material is made of.

One leading candidate is a hypothetical particle called a weakly interacting massive particle (WIMP), which would also interact with regular matter via the weak nuclear force. If so, it could show itself directly in experiments deep underground or indirectly as the glow left over when WIMPs collide and disappear in a puff of other particles.

Hooper and his colleagues first spotted the potential signal of dark matter collisions in publicly available data from NASA’s Fermi space telescope. That data showed an extra-bright gamma ray glow coming from the galactic centre, a region thought to be rich in dark matter.

In their latest paper, Hooper and his colleagues outline tests they have run to rule out gamma-ray sources that could mimic dark matter, such as fast-spinning pulsars. They also plotted the most well-characterised gamma rays on a map of the Milky Way’s disc. Subtracting known background levels, they found that extra gamma rays form a sphere around the galactic centre. The sphere’s radius extends nearly 5000 light years – further than previous measurements had been able to see and much further than you would expect from pulsars, says Hooper.

“At this point, there are no known or proposed astrophysical mechanisms or sources that can account for this emission,” he says. “That doesn’t rule out things that no one’s thought of yet, but we’ve tried pretty hard to think of something without success.”

Dark duck

Kevork Abazajian at the University of California, Irvine, thinks an unusual class of pulsars could still account for the signals. But he’s also cautiously optimistic that the dark-matter angle will pan out. “It quacks like a duck, it looks like a duck. It has all the features that you would think dark matter should have, which is remarkable,” he says. “Either we’ll find something amazing with further study, which is that it’s dark matter, or we’ll learn something new about pulsars.”

Astrophysicists have said before that seeing the same signal in dwarf galaxies would clinch the case. These smaller objects should also be full of dark matter, but they are much less dense and so the signal should be harder to spot. Now the team of scientists working directly with the NASA telescope has found tentative hints of a gamma ray excess in 25 dwarf galaxies that orbit the Milky Way. Intriguingly, their signal seems to match the one seen by Hooper’s team.

“It’s not anything that one could look at and say, ah, see, we’ve got confirmation, we have a discovery,” says Hooper. “But it’s a hint, and if you took that hint seriously, it would imply that the signal is essentially the same as you would need to explain the galactic-centre signal.”

The wrinkle is that last year the signal Hooper has been studying looked to be coming from a WIMP with a mass of about 10 gigaelectronvolts (GeV), on the lighter end of what theory predicts a WIMP should weigh. That mass fits well with hints of dark matter from underground detectors, some of which show tentative signs of similarly lightweight WIMPs.

Thermal relics

Now the signal looks like it’s coming from a 30 to 40 GeV particle, which is closer to what the simplest theories of dark matter predicted – but incompatible with the tentative findings of direct experiments. “These heavier WIMPs are the standard thermal relics of the big bang that guys like me have been looking for since we were in grad school,” says Hooper.

It is possible that LUX, the most sensitive underground detector yet, will help decide which version of a WIMP is the winner. In a separate paper, Hooper predicts that some heavier WIMPs could show up in a detector like LUX as early as next year, although some WIMPS would never show up at all.

Also, the team working with the space telescope has access to more gamma-ray data that has not been released publicly, and they are conducting their own analyses. They have yet to weigh in on whether they’re seeing the same signal as Hooper and his colleagues, but that data could be coming in the next few months.

“We’re working on an analysis of this region of the sky, and we hope to have this done very soon,” says Fermi telescope scientist Simona Murgia at UC Irvine. “Until this analysis is finished, we can’t confirm or rule out this other work. They are excellent scientists and have done very interesting work, but having different analyses producing a consistent result is a necessary step forward.”

Journal references: Physical Review D, DOI: 10.1103/PhysRevD.89.042001, arxiv.org/abs/arXiv:1402.6703 and arxiv.org/abs/1404.0022