positron
The positron is the antiparticle of the electron: it has exactly the same tiny mass and the same amount of charge, but the charge is positive instead of negative. It was the first piece of antimatter ever identified, and discovering it confirmed in the most spectacular way that the abstract mathematics of relativistic quantum theory was describing something real.
Its story runs from prediction to detection in just four years. In 1928 Dirac's equation produced puzzling negative-energy solutions, which he eventually argued must mean a positively charged twin of the electron. In 1932 Carl Anderson, studying the tracks left by cosmic rays in a cloud chamber, photographed a particle that curved like an electron in a magnetic field but in the opposite direction — exactly the signature of a positive electron.
Today positrons are commonplace and useful. They emerge from certain radioactive nuclei, and medical PET scanners rely on them: a tracer in the body emits positrons, each of which quickly meets an electron and annihilates into two photons that detectors pin down to map living tissue. The positron is thus both a historic triumph of theory and a workaday tool that quietly serves in hospitals.
Predicted by Dirac in 1928, found in cosmic rays by Anderson in 1932 — the first antimatter.
A positron is a real particle, not a hole or a 'missing electron', in the modern field-theory view; Dirac's original hole picture was a useful scaffold that has since been superseded. It is fully stable on its own and only annihilates when it meets matter.