  The universe has evolved through several phases as its various constituents
dominated its energy content. Candidate dark matter particles may have
undergone freeze-out during any such phase. While the standard freeze-out
scenarios have been explored during the radiation-dominated era, and more
recently during scalar field decay, this work extends the study of dark matter
freeze-out to a potential early period during which the universe is
matter-dominated and its evolution adiabatic. Decoupling during an adiabatic
matter dominated era changes the freeze-out dynamics, since the Hubble rate is
parametrically different for matter and radiation domination. Furthermore, for
successful Big Bang Nucleosynthesis the state dominating the early universe
energy density must decay, this dilutes (or repopulates) the dark matter. As a
result, the masses and couplings required to match the observed dark matter
relic density can differ signifcantly from radiation dominated freeze-out.
