  Weakly- and Feebly-Interacting Massive Particles (WIMPs and FIMPs) are among
the best-motivated dark matter (DM) candidates. In this paper, we investigate
the production of DM through the WIMP and FIMP mechanisms during inflationary
reheating. We show that the details of the reheating, such as the inflaton
potential and the reheating temperature, have a strong impact on the genesis of
DM. The strong entropy injection caused by the inflaton decay has to be
compensated by a reduction of the portal coupling in the case of WIMPs, or by
an increase in the case of FIMPs. We pinpoint the smooth transition between the
WIMP and the FIMP regimes in the case of low reheating temperature. As an
example, we perform a full numerical analysis of the singlet-scalar DM model;
however, our results are generic and adaptable to other particle DM candidates.
Interestingly, in the singlet-scalar DM model with low-reheating temperature,
regions favored by the FIMP mechanism are already being tested by direct
detection experiments such as LZ and XENONnT.
