Well it’s not much to write home about, and I’m not going to write about it in detail right now, but the Resonaances blog has done so (and he’s asking for your traffic, so please click):
A team of six astronomers reports that when they examine the light (more specifically, the X-rays) coming from clusters of galaxies around the sky, and account for all the X-ray emission lines [light emitted in extremely narrow bands by atoms or their nuclei] they know about, there’s an excess of photons [particles of light] with energy E=(3.55-3.57)+/-0.03 keV, a “weak unidentified emission line”, that can’t easily be explained. What could it be?
[A keV is 1000 eV; an eV is an electron-volt, an amount of energy typical of chemical reactions. Note that physicists and astronomers commonly use the word "light" to refer not just to "visible light" -- the light you can see -- but to all electromagnetic waves, no matter what their frequency. ]
Well first: is this emission line really there? The astronomers claim to detect it in several ways, but “the detection is at the limit of the current instrument capabilities and subject to significant modeling uncertainties” — in other words, it requires some squinting — so they are cautious in their statements.
Second: if it’s really there, what’s it due to? Well, the most exciting and least likely possibility is that it’s from dark matter particles decaying to a photon with the above-mentioned energy plus a second, unobserved, particle — perhaps a neutrino, perhaps something else. I’ll let Resonaances explain the sterile neutrino hypothesis, in which the dark matter particles are kind of like neutrinos — they’re fermions, like neutrinos, and they are connected to neutrinos in some way, though they aren’t as directly affected by the weak nuclear force.
But before you get excited, note that the authors state: “However, based on the cluster masses and distances, the line in Perseus is much brighter than expected in this model, significantly deviating from other subsamples.” In other words: don’t get excited, because something very funny is going on in the Perseus cluster, and until that’s understood, the data can’t be said to be particularly consistent with a dark matter hypothesis.
One more anomaly — one more hint of dark matter — to put on the pile of weak and largely unrelated hints that we’ve already got! I don’t suggest losing sleep over it… at least not until it’s confirmed by other groups and the Perseus cluster’s odd emissions are explained.