When Clusters Collide: Constraints On Antimatter On The Largest Scales
  Observations have ruled out the presence of significant amounts of antimatter in the Universe on scales ranging from the solar system, to the Galaxy, to groups and clusters of galaxies, and even to distances comparable to the scale of the present horizon. Except for the model-dependent constraints on the largest scales, the most significant upper limits to diffuse antimatter in the Universe are those on the Mpc scale of clusters of galaxies provided by the EGRET upper bounds to annihilation gamma-rays from galaxy clusters whose intra-cluster gas is revealed through its x-ray emission. On the scale of individual clusters of galaxies the upper bounds to the fraction of mixed matter and antimatter for the 55 clusters from a flux-limited x-ray survey range from < 5 x 10^(-9) to < 1 x 10^(-6), strongly suggesting that individual clusters of galaxies are made entirely of matter or, of antimatter. X-ray and gamma-ray observations of colliding clusters of galaxies, such as the Bullet Cluster, permit these constraints to be extended to even larger scales. If the observations of the Bullet Cluster, where the upper bound to the antimatter fraction is found to be < 3 x 10^-6, can be generalized to other colliding clusters of galaxies, cosmologically significant amounts of antimatter will be excluded on scales of order 20 Mpc (5 x 10^(15)M_Sun). 
Gary Steigman