What is Dark Matter?
Our galaxy is known to be surrounded by a halo of dark matter whose composition remains a mystery. Fritz Zwicky proposed in 1933 that the existence of the dark matter is a source of gravitational potential explaining rapid motions of galaxies in the Coma Cluster. Since then, evidence for dark matter has been firmly established by astrophysical and cosmological observations, such as gravitational effects on visible matter and gravitational lensing of background radiation. We now know that only 5% of the universe consists of ordinary matter, while the remainder is 27% dark matter and 68% dark energy. Identifying the nature of this dark matter is the longest outstanding problem in all of modern physics.
A leading candidate for dark matter is a Weakly Interacting Massive Particle (WIMP). These particles only interacts within the weak force, in addition to the gravity, and are electrically neutral. WIMPs are theoretically favored because they can be produced in the early universe with the correct abundance to result in the observed relic density. The expected WIMP mass ranges from 1 GeV to 10 TeV, and many experimental efforts are currently underway.