A leading hypothesis on the nature of Dark Matter is that it is comprised of Weakly Interacting Massive Particles, or WIMPs, that were produced moments after the Big Bang. If WIMPs are the dark matter, then their presence in our galaxy may be detectable via scattering from atomic nuclei in detectors as shown in this cartoon:
To shield the experiment from backgrounds from cosmic rays the detectors are located deep underground - sometimes in active (SNOLAB: 6000 feet) or inactive (Soudan: ~2300 feet) mines. However, even in these deep underground locations the detectors need to be very sensitive.
Because of the relative motion of Earth through the dark matter halo we expect an annual modulation of the dark matter signal.