As we've recently discussed at length, dark matter is likely to be a WIMP: a weakly interacting massive particle. Weakly interacting doesn't mean no interactions, though, and there's always the chance that dark matter particles will collide with something else. Since dark matter is also the most common matter in the Universe, there's a good chance that the "something else" will be another dark matter particle. And if the collision results in the destruction of dark matter particles, it should produce a spray of things we can see, like energetic particles and photons. On its own, these collisions will be too rare to detect. But summed over large regions of the sky, we might be able to detect the collective output of many collisions. This has led to a number of astronomical dark matter searches, some of which have claimed to observe puzzling excesses of high-energy photons. Yet for each one of these results, there have been other researchers who have suggested that the champagne bottles reserved for the discovery must be quietly put back to chill longer. Why so many on-again, off-again discoveries? A review published in PNAS explains why looking at high-energy photons has been so difficult, and it describes what our prospects are for making one of these discoveries stick. More via Ars Technica.