NASA's Fermi Gamma-ray Space Telescope and its Gamma-ray Burst Monitor (GBM) detected the antimatter, beams shooting above thunderstorms. The phenomenon has never been seen before. Here's what NASA thinks is happening:
Scientists think the antimatter particles were formed in a terrestrial gamma-ray flash (TGF), a brief burst produced inside thunderstorms and shown to be associated with lightning. [...]This discovery means that starship commanders won't have to head over to CERN to borrow (take) some of their antimatter from the Large Hadron Collider.
Fermi is designed to monitor gamma rays, the highest energy form of light. When antimatter striking Fermi collides with a particle of normal matter, both particles immediately are annihilated and transformed into gamma rays. The GBM has detected gamma rays with energies of 511,000 electron volts, a signal indicating an electron has met its antimatter counterpart, a positron.