While watching the tracks of cosmic ray particles passing through his cloud chamber, Carl Anderson discovered antimatter in the form of the antielectron, later called the positron. A positron is a particle exactly like an electron, but with an opposite, positive charge.
A debate raged over the nature of cosmic rays. According to a theory of Robert Millikan, they were gamma rays from space -- hence the name "cosmic rays." But evidence was mounting that cosmic rays were, in fact, mostly energetic particles.