Intergalactic Mystery X-Rays Point to Dark Matter

​Astronomers parsing data collected by the European Space Agency's XMM-Newton spacecraft have come up with an unexpected bump in the x-ray emission rates from the Andromeda galaxy and the Perseus galaxy cluster. Simply: it's known what given celestial objects should be radiating out into the cosmos based on understood (observed) emission sources, and yet there is still something extra.

This is dark matter, potentially.

First, a quick dark matter recap: DM is the stuff that, based on its gravitational effects on galaxies, we can estimate to be about 26 percent of everything in the universe (compared to 4 percent "normal" matter with the rest being dark energy). It's usually assumed to be some sort of particle that interacts poorly with non-dark matter, experiencing perhaps fewer or none of the fundamental forces beyond gravity.

Most detection experiments on Earth (and there are quite a lot), look for particles called WIMPs, which are heavy and weakly-interacting (experiencing mostly just the gravitational force). But some other theories suggest particles that are very, very light and experiences the same forces, but just really timidly. As the WIMP experiments continue to come up dry, more attention has recently become focused on the latter idea of light-matter "topological defects." (These cosmic fogs of light matter condensate are also the more interesting possibility, IMO.)

A detector aboard the International Space Station has also recorded a bit of an extra something in the cosmic rays smashing into Earth's atmosphere, another discrepancy between predictions based on known phenomenon and observations. Neither those observations or XMM-Newton's x-ray bump offer dark matter on a on a fishing hook, but they certainly raise eyebrows.

The new findings, which are set to be published in the Physical Review Letters, align with yet another dark matter theory: sterile neutrinos. These neutrinos are a lot like the neutrinos we known and love—cosmic drifters that don't interact electromagnetically, like uncharged electrons—except they don't even feel the weak force, which is the one that has to do with radioactive decay.

Sterile neutrinos should decompose into x-rays, which brings us back to the anomalous signals. "The relative fluxes for the two objects are in agreement with what is known about their DM distributions," the astronomers write in a pre-print version of the upcoming paper.

"The upper bound from this dataset is consistent with expectations for a DM signal that would come in this case primarily from the Milky Way halo," the report continues. "However, as the line is weak and the uncertainties in DM distribution are significant, positive detections or strong constraints from more objects are clearly needed in order to determine the nature of this signal."

So, what now?

"Confirmation of this discovery may lead to construction of new telescopes specially designed for studying the signals from dark matter particles," Alexey Boyarsky, a physicist based at Leiden University, offered in a statement. "We will know where to look in order to trace dark structures in space and will be able to reconstruct how the universe has formed."