A mile beneath Italy's Gran Sasso mountain lies the DarkSide-50 detector. The three-story cylinder was built to search for our universe's most mysterious substance: dark matter. "We know it exists in our galaxy and roughly how much there is," says Princeton physicist Peter Meyers. "What we don't know is what it is." The most promising lead is WIMPs--weakly interacting massive particles. If they do exist, these theoretical particles should drift through the walls of DarkSide-50's three nested tanks and collide with atoms of liquid argon at its core. The argon atoms would then recoil like billiard balls and emit light, providing proof of WIMPs and bringing us closer to figuring out dark matter's elusive identity.