Antimatter in the Universe

Balloon and satellite experiments detect antimatter in the Universe both directly, as a cosmic ray component, and indirectly, through characteristic annihilation signatures.
Positrons: For over five decades positrons have been studied in cosmic rays, just as long as we have been observing them through the 511 keV gamma-rays they emit when annihilating. The question on their origin, however, is still far from being settled, neither for the high-energy cosmic ray positrons, nor for the positrons annihilating in the central regions of our Galaxy. Nevertheless, the most plausible scenarios for their production all involve processes occurring at the endpoints of stellar evolution.
Baryonic antimatter: During forty years, direct detection of baryonic antimatter has essentially concerned the measurement of antiprotons, naturally produced in cosmic ray interactions with the interstellar medium. Recently, the tentative detection of a few 3He by AMS has revitalized the discussion on the existence of baryonic antimatter in the Universe. Since revoking an ”MeV bump” hinted in the seventies, gamma-ray astronomy has more and more constrained the fraction of antimatter possibly contained in astrophysical objects. The absence of characteristic annihilation features on all scales has virtually ruled out the existence of substantial quantities of antimatter in the observable Universe.