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 of the Big Bang, similar to the cosmic microwave background. In the present epoch these neutrinos play no strong role in the evolution of the Universe, but some of them may be the reason we live in a matter-dominated universe.

One of the basic symmetries of the Universe is that every elementary particle has a twin. Electrons have the positron. Quarks have anti-quarks.…