Although NASA's Fermi Gamma-ray Space Telescope is designed to observe high-energy events throughout the universe, it sometimes detects events on Earth. Scientists using Fermi have detected beams of antimatter produced in thunderstorms on Earth, a phenomenon never before seen. 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. It is estimated that about 500 TGFs occur daily worldwide, but most go undetected.