From NASA Science News:
The spectral line appears not to come from any known type of matter, which shifts suspicion to the unknown: dark matter.
“After we submitted the paper, theoreticians came up with about 60 different dark matter types which could explain this line. Some particle physicists have jokingly called this particle a ‘bulbulon’,” she laughs.
The menagerie of dark matter candidates that might produce this kind of line include axions, sterile neutrinos, and “moduli dark matter” that may result from the curling up of extra dimensions in string theory.
Mysterious signal from the center of the Perseus Cluster unexplained by known physics
“I couldn’t believe my eyes,” says Esra Bulbul of the Harvard Center for Astrophysics. “What we found, at first glance, could not be explained by known physics.”
“The cluster’s atmosphere is full of ions such as Fe XXV, Si XIV, and S XV. Each one produces a ‘bump’ or ‘line’ in the x-ray spectrum, which we can map using Chandra. These spectral lines are at well-known x-ray energies.”
Yet, in 2012 when Bulbul added together 17 day’s worth of Chandra data, a new line popped up where no line should be.
“A line appeared at 3.56 keV (kilo-electron volts) which does not correspond to any known atomic transition,” she says. “It was a great surprise.”
“It took a long time to convince myself that this line is neither a detector artifact, nor a known atomic line,” Bulbul said. “I have done very careful checks. I have re-analyzed the data; split the data set into different sub groups; and checked the data from four other detectors on board two different observatories. None of these efforts made the line disappear.”