Anybody care to give a layman/HS explanation of why this might be dark matter?
It's been a long time since High School chemistry class, but as I remember it, spectral emission lines are caused by electrons changing orbital levels and thereby releasing a photon. So dark matter has electrons? Er, would that mean that it's not dark? Why wouldn't this just mean that we've missed an orbital level somewhere?
The authors of the study are claiming it as an emission line, but not all emission lines come from transitions of electrons. Some come from changes in quantum rotation states (e.g., some molecular transitions), rotation-vibration states (other molecular transitions), or flips in the spin direction of electrons (the famous hydrogen 21cm line). In this case, the authors have not found known lines from molecules or atoms which would correspond to this energy.
The suggestion of dark energy comes from some theories where postulated dark matter particles can annihilate if they hit another dark matter particle. In these theories, the result is emission of high energy photons with an energy which is related to the mass of the individual dark matter particle. Basically E=mc^2. So it is still an emission line, but not one that arises from an election transition in an atom or molecule.
Yes, if it interacts with photons it means that it's not "dark". But the problem is that atomic physics is awfully well travelled, the chances that we've "missed an orbital" somewhere in a conventional material seems slim (or at least it does to the authors -- I'm not an expert).
So a novel photon interaction in intergalactic space would be huge news even if it doesn't map directly to dark matter.
There is lots of unexplored territory in molecular spectroscopy. The ALMA Telescope[0]—primarily operating at millimeter and sub-millimeter wavelengths, where the bulk of molecular transitions are—is likely to discover many spectral lines which have not been classified. Estimates suggest that 50–80% of the lines seen in some observations will correspond to unknown transitions of known molecules or transitions from previously unknown molecules. But the energies of molecular (rotational) transitions are generally far too low for an unknown molecular species to be a viable explanation for a spectral line in the X-rays.
Not perfectly 'dark' or we wouldn't see this at all. Still makes it dark matter for all practical purposes. Given the absolutely massive amount of dark matter that is out there the fact that there's a tiny emission line hinting at its existence that has taken us decades to find indicates that its still pretty damn dark (assuming this holds up).
It's been a long time since High School chemistry class, but as I remember it, spectral emission lines are caused by electrons changing orbital levels and thereby releasing a photon. So dark matter has electrons? Er, would that mean that it's not dark? Why wouldn't this just mean that we've missed an orbital level somewhere?