The dynamics of galaxies and clusters of galaxies points towwards the existence of dark matter. Different dark matter candidates do exist. direct searches for dark matter focus on so-called WIMPs, Weakly Interacting Dark Matter. These particles do not exist in the standard model of particle physics and do not only interact gravitationally, but also weakly.
Direct dark matter experiments look for interactions between WIMPS and the nuclei of the detector. Since the interaction between the nuclei and the WIMP is very weak, the experiments are performed in underground laboratories, in order to screen from cosmic radiation and natural radioactivity.