We might have just seen the first hints of dark matter

An unexplained glow that appears to be coming from the outer regions of the Milky Way may be our first hint of what dark matter is made of, but astronomers say it's too early to say for sure.

Dark matter is believed to consist of 85 percent of the total mass of the Universebut physicists have never been able to detect the particles that make it up.

One of the leading candidates for dark matter is a ghostly object called a weakly interacting massive particle (WIMP). These hypothetical particles are damn hard to detect because they interact very rarely with ordinary matter, but theorists predict that they should self-destruct from time to time, disappearing and producing a burst of high-energy radiation in the form of gamma rays.

If dark matter is distributed throughout our galaxy, as its gravitational pull suggests, and it also consists of WIMPs, then we should see the glow of self-destructing WIMPs. Astronomers argued There has been debate for over a decade over whether a strange excess of gamma rays from the center of our galaxy could be this signal, but the evidence is still inconclusive.

Now, Tomonori Totani from the University of Tokyo says he may have detected such a signal coming from the outer part of the Milky Way, known as its halo, using 15 years of observations from NASA's Fermi Gamma-ray Space Telescope.

Totani first developed a model of how much gamma rays there should be in the region, based on known sources such as stars, cosmic rays and large bubbles of radiation observed above and below the Milky Way. He then subtracted this radiation from the amount observed by the Fermi telescope and discovered a residual gamma-ray glow with an energy of about 20 gigaelectronvolts.

A gamma ray signal with that energy is consistent with what might come from a self-destructing particle in the energy range that WIMPs are predicted to have, Totani said. While he admits it's too early to definitively conclude that the gamma-ray burst is coming from dark matter, he says the signal is “the most promising candidate for dark matter emission known to date.”

“Even though the research started with the goal of detecting dark matter signals, I thought it was like playing the lottery. So when I first noticed what seemed to be a signal, I was skeptical,” says Totoni. “But when I took the time to thoroughly test it and felt confident that it was right, I got goosebumps.”

“This result certainly deserves further study, but it would be premature to draw firm conclusions now,” says Francesca Calor at the French National Center for Scientific Research in Annecy. It's difficult to accurately model all gamma-ray sources in the Milky Way other than dark matter, and Totoni hasn't rigorously stress-tested the models, she said.

Silvia Manconi from the Sorbonne University in France agrees that the results have not been thoroughly tested and we will need more complex models to accurately say whether the signal is real. Additionally, she said, we haven't seen these gamma-ray signals from other sources where they should be, such as dwarf galaxies, so the discrepancy needs to be explained.

We'll need to look at a variety of other radiation sources, such as radio waves and neutrinos, to make sure the gamma rays aren't coming from something else, he says. Anthony Brown at Durham University, UK. “It's just one angle,” he says. “Dark matter really needs as much high-quality data as possible.”

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