When a massive star dies, its core collapses, and a supernova explosion occurs. The cataclysm births streams of particles — like ripples from a thrown stone. The first to arrive are swift neutrinos, which pierce the Earth, hardly interacting. Then, hours later, ghost particles with a tiny charge appear.
Underground detectors catch faint flashes from these particles colliding with electrons. Because of their mass, they travel slower than neutrinos and arrive later, like an echo. Detectors could register up to tens of events per year — dozens of times more than previous estimates. Perhaps these ghost particles are dark matter — the invisible scaffolding of the Universe.
Neutrinos from supernova 1987A outpaced light by three hours — proving they have a tiny mass.