  The relic abundance of thermal dark matter particles is generally assumed to
be inversely proportional to their annihilation rate, which is therefore
constrained by the present matter density, <sigma v> ~ 10^{-26} Omega_{dm}^{-1}
cm^3 sec^{-1}. Here we point out that much lower values of <sigma v> are
possible for heavy dark matter candidates (m < 10 TeV) that couple to other
particle species through the electroweak force. With heavy dark matter
particles present the early universe may evolve according to the following
scenario. After an early entry into matter-dominated phase, dark matter
particles form self-gravitating microhalos. Collisional interaction between
dark matter particles and the surrounding radiation field eventually leads to
microhalos gravothermal collapse and annihilation of most dark matter
particles. For sufficiently heavy dark matter candidates (m < 10 TeV) the
universe can return to radiation-dominated phase before the nucleosynthesis and
thereafter follow the "standard" scenario.
