In this program, NOVA probes the secret ingredient of the cosmos: swarms of invisible particles that fill every cubic inch of space and just may explain how the universe was created. Trillions of ghostly neutrinos move through our bodies every second without us noticing a thing. Yet without them the sun wouldn't shine and the elements that make up our world wouldn't exist. This program explores the 70-year struggle so far to understand the most elusive of all elementary particles, the neutrino.