In cosmology, the relic abundance of a given elementary particle is a measure of the present quantity of that particle remaining from the Big Bang.
Relic abundance is modelled for WIMPs (weakly interacting massive particles) in the study of dark matter.
[1] Assuming that an elementary particle was formerly in thermal equilibrium, its relic abundance may be calculated using a Boltzmann equation.
[2] The temperature scaled abundance of a particle is defined[3] by where
indicates the asymptotic value of abundance of a species of a particle which it will reach after its "freeze-out".