This question straddles this forum and the "Beyond the Standard Model" one a bit, so if a mentor thinks it belongs better elsewhere, please feel free to move it. I've seen references in the "popular science" press about the possibility of indirectly detecting dark matter by looking for gamma rays produced in annihilation. This is a process that seems to be expected specifically with WIMPs and possibly with other models. For example, here. I know pop sci press can be misleading, but the literature seems to agree, e.g. arXiv:1203.5636 and arXiv:1303.3284 What I don't understand is how dark matter particles annihilating each other can produce gamma rays. That would require a vertex in the Feynman rules linking the dark matter field with the electromagnetic potential, which in turn would contradict the basic assumption about dark matter: that it doesn't interact electromagnetically. How do you get gamma rays out of something that, by definition, isn't connected to the EM field? My first guess would have been that the dark matter particles are not producing gamma rays directly but that their annihilation products scatter off of regular matter and produce high energy photons. This doesn't seem to be the case, though. According to this Stanford research group: "In some of these models, the dark matter particle may self-annihilate or decay into standard model particles, including photons with energies as large as the dark matter particle rest mass." So it seems like the expectation is that DM particles can produce gamma rays directly. How is this possible?