  I argue that the existence of cold antimatter in bulk is not permitted by the
Standard Model, so that if a gamma-ray signature from antiproton annihilation
were to be detected, it must represent either new physics or the action of
intelligence. Time variability of the signal would strongly support the second
alternative. The entire sky was scanned at the relevant energies (30- 928 MeV)
by the EGRET experiment on board the Compton Gamma Ray Observatory during
1991-1995. A search of this database for the antiproton annihilation signature
yielded only upper limits on the flux (an intriguing spectrum detected from QSO
2206+650 = 3EG J2206+6602 is probably not related to SETI). The all-sky,
longterm 99 upper limit is 2.3 x 10^{-8} photon/(cm2 s); it is a factor 10
worse in the Galactic plane due to the higher diffuse gamma-ray background
emission. I give brief, but quantitative, illustrations of what this limit
means for nearby intelligent activities.

Astrophysics