Most of the universe is made up out of things we can't find. While dark matter is just that — dark — an X-ray anomaly in Perseus and 72 other galaxies may finally be a tell-tale signature of a particular form of dark matter releasing light as it decays.
A strange X-ray emission line shows up when looking at 73 galaxy clusters, a spike of intensity centered at the 3.56 keV energy level. It doesn't match up with the emission lines we expect from the normal collection of gas and dust that make up a galaxy. The wavelengths were something new, something different... and anything weird is very exciting for scientists.
Image Credit: Chandra: NASA/CXC/SAO/E.Bulbul, et al.; XMM: ESA
It's going to take more observations to confirm that the spike really exists, and to figure out its nature. But in the meantime, it's certainly fun to come up with theories as to what is causing it. If it isn't anything ordinary from the matter we see all the time, then it needs to be from something special, dark matter.
If the spike really exists, it could be the marker of a particular form of dark matter, sterile neutrinos. Normal neutrinos interact with matter with the weak nuclear force and gravity, but sterile neutrinos only interact with matter with gravity. At the moment, sterile neutrinos are a subatomic particle that have been predicted but not yet detected, so they might not even exist. But theoretically, the decay of sterile neutrinos could produce X-rays matching the signal of what we think we've observed in Perseus.
Except that the observations don't quite match theory: it's within the upper limits of acceptable, but is far brighter than we think it should be. This could mean: 1. that we're a bit off on the properties of this subatomic particle we haven't seen yet; 2. that source of the X-rays are something other than sterile neutrinos; or 3. that the apparent X-ray anomaly is the result of some human or instrument error. My favourite explanation for the mismatch is that it could be a particular dielectronic recombination line, but then the emissivity is 30 times greater than expected, a situation the researchers describe as, "physically difficult to understand." Isn't that just a delightfully understated way of admitting we don't know what's going on?
Perseus and 72 other galaxies have an anomalous X-ray signature, a spike at 3.56 keV. Image credit: NASA/CXC/SAO/E.Bulbul, et al.
The signal is popping up in galaxy clusters near and far, from a hundred million light-years to a few billion light-years. It's going to take more observations from XMM-Newton, Chandra, and other high-energy telescopes to confirm that the anomaly is real.
In the meantime, ESA's XMM-Newton Project Scientist Norbert Schartel confides, "It would be extremely exciting to confirm that XMM-Newton helped us find the first direct sign of dark matter. We aren't quite there yet, but we're certainly going to learn a lot about the content of our bizarre Universe while getting there."