In the first millionths of a second after the Big Bang, the universe was a roiling, trillion-degree plasma of quarks and gluons — elementary particles that briefly glommed together in countless combinations before cooling and settling into more stable configurations to make the neutrons and protons of ordinary matter. In the chaos before cooling, a fraction of these quarks and gluons collided randomly to form short-lived “X” particles, so named for their mysterious, unknown structures. Today, X particles are extremely rare, though physicists have theorized that they may be created…
