Neutrinos don’t interact strongly with regular matter. Countless neutrinos stream through us all the time with no effect. But neutrinos weren’t always so weakly interacting. In the earliest cosmic moments neutrinos still interacted strongly with protons, neutrons, and electrons. All of these particles were in thermal equilibrium for a time until the Universe could expand to a critical point. At about a second after the Big Bang neutrons decoupled from other material particles. They began to act similar to the neutrinos we see today. These decoupled neutrinos still fill the Universe. They form a cosmic neutrino background. A particle echo...