  Neutrinos heavier than $M_Z/2\sim 45$ GeV are not excluded by particle
physics data. Stable neutrinos heavier than this might contribute to the cosmic
gamma ray background through annihilation in distant galaxies as well as to the
dark matter content of the universe. We calculate the evolution of the heavy
neutrino density in the universe as a function of its mass, $M_N$, and then the
subsequent gamma ray spectrum from annihilation of distant $N\bar{N}$ (from
$0<z<5$). The evolution of the heavy neutrino density in the universe is
calculated numerically. In order to obtain the enhancement due to structure
formation in the universe, we approximate the distribution of $N$ to be
proportional to that of dark matter in the GalICS model. The calculated gamma
ray spectrum is compared to the measured EGRET data. A conservative exclusion
region for the heavy neutrino mass is 100 to 200 GeV, both from EGRET data and
our re-evalutation of the Kamiokande data. The heavy neutrino contribution to
dark matter is found to be at most 15%.
