A couple of years ago, astronomers took note of a curious x-ray signal in data from both NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton Observatory. First, it was seen in observations of the Perseus galaxy gathered by both instruments, and, then, it was all over the place. XMM-Newton found the same signal present in 73 other galaxies: an x-ray emission line at a frequency where there shouldn't be one. Thoughts quickly turned to a dark matter explanation.
Of course, dark matter being dark and all sort of precludes it from spitting out x-rays. Instead, the idea is that as certain dark matter particles decay, they might emit x-rays. This is the case for one potential dark matter explanation: sterile neutrinos. These hypothesized particles interact only via gravity, a la dark matter, but as they decay, they should emit an x-ray glow. We haven't actually detected sterile neutrinos yet owing to the extreme sensitivities required. Indeed, the x-ray signals observed by Chandra and XMM-Newton were at the upper limits of their observational powers.
So, as it turns out, the signals may not be so dark or mysterious. According to researchers at the Max Planck Institute for Nuclear Physics in Heidelberg, they are likely the product of highly-charged sulfur ions being devious. This is all described in a paper posted recently to the arXiv pre-print server and slated for publication in the Astrophysical Journal.
In the hot medium often found between galaxies of the same cluster it's common to find ionized atoms—atoms that have been stripped of their electrons such that they're just naked, positively-charged nuclei. These nuclei like to steal electrons from other particles, including co-mingling hydrogen atoms. So, the sulphur ions suck up these electrons and then spit them back as x-rays.
The Max Planck scientists tested this out in the lab by trapping some sulphur atoms in a vacuum and then bombarding them with electrons. The electrons then knocked other electrons out of the sulphur atoms, leaving charged ions. The physicists then turned off the beam and watched what happened as the ions swiped energetic electrons from other free-floating molecules. The result was an emission of x-rays at energies of around 3.47 kiloelectronvolts. This is very near what was seen by Chandra and XMM-Newton.
"This intriguing X-ray line feature arises from charge exchange between fully stripped sulfur ions and atomic hydrogen," the Max Planck team reports.
This isn't some huge setback in dark matter research, generally. The explanation provided in the current paper was predicted by another group of astrophysicists last year. There was also the issue of the observed x-ray emission line not really matching known dark matter distributions across space. Still, as various dark matter hunts continue to come up empty handed, this is open-season for fringe explanations. Keem em' coming.