I get what you are saying but.... if it can decay into a photon isn't that the same as emitting a photon? The first line quoted above is "but does not emit or absorb light like “normal” matter." If it can generate a Photon (by decay or someother means) it can't be dark. I'm not that smart but Dark Mattter/Energy thing just stinks of take a stab in the dark (no pun intended). I think some where we have some constant of the universe calculated wrong, or we are estimating the mass of the galaxies wrong.