A quarter of the stuff in our universe is unaccounted for. We see its effects written across the heavens, but it has never been seen and cannot be explained. This mysterious substance is known as dark matter. Scientists hope that, by discovering the identity of dark matter, they will discover an essential piece in the cosmic jigsaw, as well as answer some problems with the standard model of particle physics. From CERN in Switzerland to a telescope in Chile, Horizon meets the scientists trying to solve one of the biggest mysteries in the universe and asks what success or failure will mean for the future of physics.