Neutrino mass and the origin of matter

Unless you work in a specialized physics lab, you may never confront antimatter. All the stuff you normally deal with is made of protons, neutrons, and electrons—regular matter, not antimatter. In other respects, matter and antimatter are quite similar; so why do we encounter essentially only matter? Perhaps the universe has no cosmic matter-antimatter asymmetry, but we just happen to live in a matter neighborhood. Suppose, then, that the universe started with equal amounts of matter and antimatter. Early in its evolution, the universe’s energy was squeezed into a hot plasma in a very tiny volume. Under such conditions, reactions in the plasma would maintain matter-antimatter equality. Hence, equal amounts of matter and antimatter would be “frozen out” when the universe had expanded sufficiently that would-be colliding matter and antimatter had difficulty finding one another to annihilate. Well-understood physics allows theorists to calculate the residual amount of matter and antimatter;...