Plot: What happened to all of the universe's antimatter? Can a particle be its own anti-particle? And how do you build an experiment to find out? In this program, particle physicists reveal their hunt for a neutrino event so rare, it happens to a single atom at most once every 10,000,000,000,000,000,000,000,000 years: far longer than the current age of the universe. If they find it, it could explain no less than the existence of our matter-filled universe