They are the most abundant particles in the universe, yet we barely know they exist. Neutrinos stream through everything, through walls, through planets and even through you…. in their billions every second, leaving no trace. We've known for decades that they have mass, but pinning down exactly how much has defeated physicists for years. Now, the most sensitive experiment ever built has pushed our knowledge to a new frontier, and what it found raises a profound question about why these ghostly particles are so extraordinarily light.