The WIMP Coincidence
Given the expected mass of WIMPs and the strength of their interactions, which govern how often they annihilate one another, physicists can easily calculate how many WIMPs should be left over. Rather amazingly, the number matches the number required to account for cosmic dark matter today, within the precision of the mass and interaction-strength estimates. This remarkable agreement is known as the WIMP coincidence. Thus, particles motivated by a century-old puzzle in particle physics beautifully explain cosmological observations.
This line of evidence, too, indicates that WIMPs are inert. A quick calculation predicts that nearly one billion of these particles have passed through your body since you started reading this article, and unless you are extraordinarily lucky, none has had any discernible effect. Over the course of a year you might expect just one of the WIMPs to scatter off the atomic nuclei in your cells and deposit some meager amount of energy. To have any hope of detecting such events, physicists set their particle detectors to monitor large volumes of liquid or other material for long periods. Astronomers also look for bursts of radiation in the galaxy that mark the rare collision and annihilation of orbiting WIMPs. A third way to find WIMPs is to try to synthesize them in terrestrial experiments.