Dark matter is the mysterious stuff that makes up about 83 percent of the matter in the universe, but doesn’t interact with electromagnetic forces. Although the universe contains five times as much dark matter as normal matter, dark matter is completely invisible both to human eyes and every kind of telescope ever devised. Physicists only know it’s there because of its gravitational effect on normal matter. Dark matter keeps galaxies spinning quickly without flying apart and is responsible for much of the large-scale structure in the universe.
Current dark matter detectors are looking for WIMPs, or weakly interacting massive particles, that connect only with the weak nuclear force and gravity. Based on the most widely accepted theories, most experiments are tuned to look for a particle that is about 100 times more massive than a proton. The chief suspect is also its own antiparticle: Whenever a WIMP meets another WIMP, they annihilate each other.