Constraints on ultracompact minihalos from extragalactic {\gamma}-ray
  background
  It has been proposed that ultracompact minihalos (UCMHs) might be formed in
earlier epoch. If dark matter consists of Weakly Interacting Massive Particles
(WIMPs), UCMHs can be treated as the {\gamma}-ray sources due to dark matter
annihilation within them. In this paper, we investigate the contributions of
UCMHs formed during three phase transi- tions (i.e., electroweak symmetry
breaking, QCD confinement and e+ e- annihilation) to the extragalactic
{\gamma}-ray background. Moreover, we use the Fermi-LAT observation data of the
extragalactic {\gamma}-ray background to get the constraints on the current
abundance of UCMHs produced during these phase transitions. We also compare
these results with those obtained from Cosmic Microwave Background (CMB)
observations and find that the constraints from the Fermi-LAT are more
stringent than those from CMB
