Antimatter in the Universe : Constraints from Gamma-Ray Astronomy
  We review gamma-ray observations that constrain antimatter - both baryonic
and leptonic - in the Universe. Antimatter can be probed through ordinary
matter, with the resulting annihilation gamma-rays providing indirect evidence
for its presence. Although it is generally accepted that equal amounts of
matter and antimatter have been produced in the Big Bang, gamma-rays have so
far failed to detect substantial amounts of baryonic antimatter in the
Universe. Conversely, positrons are abundantly observed through their
annihilation in the central regions of our Galaxy and, although a wealth of
astrophysical sources are plausible, their very origin is still unknown. As
both antimatter questions - the source of the Galactic positrons and the baryon
asymmetry in the Universe - can be investigated through the low energy
gamma-ray channel, the mission concept of a dedicated space telescope is
sketched out.
