A controversial X-ray glow didn’t show up in the Milky Way’s dark matter halo

Glowing X-rays around distant galaxies can’t come from dark matter particles, new research shows. The mysterious glow failed to show up in the dark matter halo around the Milky Way, astrophysicists report in the March 27 Science.

“It seems to be the final nail in the coffin of the dark matter interpretation,” says astroparticle physicist Ben Safdi of the University of Michigan in Ann Arbor.

The tantalizing glows were first detected in 2014 as an excess of X-rays with an energy of 3.5 kiloelectron volts, or keV, coming from faraway galaxy clusters. Some astronomers argued that the X-rays could come from decaying particles of dark matter — the ubiquitous, inert material that makes up more than 80 percent of the universe’s matter. The X-rays’ energy, in particular, suggested that the light could be coming from hypothetical dark matter particles called sterile neutrinos (SN: 6/1/18).

But searches for a similar X-ray glows came up empty in the dwarf galaxy Draco in 2015 (SN: 12/11/15) and the Perseus galaxy cluster in 2016 (SN: 8/12/16). So Safdi and his colleagues looked closer to home. The Milky Way’s starry spiral disk is surrounded by a halo of dark matter (SN: 3/23/20). If the X-rays come from decaying dark matter, our galaxy’s halo should sparkle with the light.

All images of the sky look through some portion of the Milky Way’s dark matter halo, even if that’s not what a telescope is aiming at. So the researchers sifted through every observation that the European Space Agency’s XMM-Newton space telescope ever made, about 347 days of total exposure time, in search of X-rays at the right energy. The team saw none.

“The jury is still out on what is producing the 3.5 keV” signal, Safdi says. “What’s clear, though, is it’s not dark matter.”