Upper bounds on matter-antimatter admixture from gamma-ray observations of colliding clusters of galaxies with the Fermi Large Area Telescope

We examined whether significant constraints on matter-antimatter admixture can be derived from gamma-ray observations of colliding clusters of galaxies with Fermi-LAT. We selected ten known systems of colliding clusters of galaxies for the analysis and computed the upper bounds on matter-antimatter admixture in these systems, which range from 7×10−9 to 2×10−6. This allowed us to exclude a symmetric universe on scales of order ∼20 Mpc at the confidence level of 99.9%. Adopting the number of systems of colliding galaxy clusters from the Marenostrum Universe cosmological simulation, we checked if the Fermi-LAT second source catalog contains a sufficient number of gamma-ray sources to provide us with the required number of sources possibly associated with pp¯annihilation from cluster-anticluster collisions. We found that a matter-antimatter-symmetric universe is strongly ruled out on scales of order ∼20 Mpc if a matter-antimatter admixture in these bullet-like systems is of f≳10−5, and on scales of order ∼400 Mpc if f≳10−4.