"Sakharov published other papers in cosmology. ...[T]he most far-reaching, innovative, and original... concerned "". "s"... denote collectively not only protons and neutrons but also... unstable particles... created when protons and neutrons collide at extraordinarily high speeds. "Antibaryons"... carry the opposite electrical charge. When baryons and antibaryons collide, they annihilate each other, producing... exotic, unstable particles, such as pi-mesons, which are lighter than baryons, as well as radiation... "quanta"... or photons, which have no mass at all. The "background radiation" cosmologists discovered in the mid-1960s is... a remnant of the... annihilation of baryons and antibaryons... when the universe was created or shortly afterward. Baryons and antibaryons, in other words, are one form of matter and antimatter, respectively; electrons... and their opposite, positrons, are another."