  Dark matter, if represented by a $\mathbb{Z}_2$-symmetric scalar field, can
manifest as both particles and condensates. In this paper, we study the
evolution of an oscillating homogeneous condensate of a
$\mathbb{Z}_2$-symmetric scalar field in a thermal plasma in an FLRW universe.
We focus on the perturbative regime where the oscillation amplitude is
sufficiently small so that parametric resonance is inefficient. This
perturbative regime necessarily comprises the late stage of the condensate
decay and determines its fate. The coupled coarse-grained equations of motion
for the condensate, radiation, and spacetime are derived from first principles
using nonequilibrium quantum field theory. We obtain analytical expressions for
the relevant microscopic quantities that enter the equations of motion and
solve the latter numerically. We find that there is always a nonvanishing relic
abundance for a condensate with a $\mathbb{Z}_2$ symmetry that is not
spontaneously broken. This is because its decay rate decreases faster than the
Hubble parameter at late times due to either the amplitude dependence or the
temperature dependence in the condensate decay rate. Consequently, accounting
for the condensate contribution to the overall dark matter relic density is
essential for $\mathbb{Z}_2$ scalar singlet dark matter.
