What is Dark Matter
A wealth of observations, dating back 70 years, show that the universe is composed of >96% invisible matter and energy. The nature of these missing components is one of the most fundamental mysteries in physics, and has attracted broad attention from the public. The leading candidate for the invisible dark matter is a subatomic particle left over from the big bang known as the Weakly Interacting Massive Particle (WIMP). Such particles are also predicted by supersymmetry, a favored class of new particle models. If WIMPs exist, they are also the dominant mass in our own Milky Way. Though they only very rarely interact with conventional matter, they should nonetheless be detectable by sufficiently sensitive detectors on Earth, through their direct interaction with, and the ensuing recoil of, nuclei in a target material. The primary challenge in detecting them is reducing natural and cosmogenic radioactivity by up to 10 orders of magnitude.