By now three kinds of quarks were known: up, down, and strange. But the theory predicted that quarks should exist in pairs, with up-down and strange-charmed proposed as a pairing. Using the latest particle accelerators, a hadron that had to contain a charmed quark was created. The third pair of quarks is the top-bottom, for a total of six quarks and six antiquarks (matching the number of leptons)
A visual chronology of Asimov's ~1500 scientific inventions and discoveries.
