We may have just seen the first hints of dark matter

An unexplained glow that appears to emanate from the outer regions of the Milky Way could be our first clue to the composition of dark matter, but astronomers say it’s too early to be sure.

Dark matter is thought to make up 85% of the total mass of the universe, but 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 extremely difficult to detect because they interact so rarely with ordinary matter, but theorists predict that they should occasionally self-annihilate, disappear, and produce a flash 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 is also composed of WIMPs, then we should see a glow of the WIMPs self-annihilating. Astronomers have debated for more than a decade whether a strange excess of gamma radiation coming from the center of our galaxy could be that signal, but the evidence is still inconclusive.

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

Totani was the first to produce a model of the amount of gamma radiation that should exist in this region, based on known sources, such as stars, cosmic rays, and large bubbles of radiation observed above and below the Milky Way. Then he subtracted this radiation from the amount seen by the Fermi telescope, finding that a gamma-ray glow remained with an energy of about 20 gigaelectron volts.

A gamma ray signal with this energy matches what could come from a self-annihilating particle in the energy range that WIMPs should have, Totani says. While he admits that it is too early to definitively conclude that the gamma ray peak comes from dark matter, he says that this signal is “the most promising candidate radiation known to date.”

“Even though the search began with the goal of detecting dark matter signals, I thought it was like playing the lottery. So when I first spotted what looked like a signal, I was skeptical,” says Totoni. “But when I took the time to check it meticulously and was sure it was correct, I got goosebumps.”

“This is a result that certainly deserves further study, but it would be premature to draw definitive conclusions now,” says Francesca Calore of the National Center for Scientific Research in Annecy. It’s difficult to accurately create a model of all the Milky Way’s gamma-ray sources other than dark matter, she says, and Totoni hasn’t tested the models extensively.

Silvia Manconi of the Sorbonne University in France agrees that the results have not been tested exhaustively and that we would need more sophisticated models to really tell if the signal is real. Additionally, we haven’t seen such gamma-ray signals from other sources where we should have had them, like dwarf galaxies, she says, so this discrepancy should be explained.

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

CERN and Mont Blanc, dark and frozen matter: Switzerland and France

Prepare to be blown away by CERN, Europe’s center for particle physics, where researchers operate the famous Large Hadron Collider, nestled near the charming Swiss lakeside town of Geneva.