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.
So a novel photon interaction in intergalactic space would be huge news even if it doesn't map directly to dark matter.