As we learned last week in Discovering Dark Matter, since the 1930s scientists have been seeking answers about unseen mass in the universe. We know that the gravitation of dark matter has an enormous effect on galaxies, and we also know that it may be made up of weakly interacting particles. But how do researchers search for something that's invisible? James Buckley, professor of physics at Washington University in St. Louis, has spent part of his career hunting for neutralinos, a yet-undiscovered type of particle that may hold the answer to the dark-matter mystery. Buckley describes the evidence for the existence of neutralinos, the methods he uses to seek them out, and how he first became interested in the "dark and violent" side of the universe.