  The initial density of dark matter (DM) particles, otherwise secluded from
the standard model (SM), may be generated at reheating, with an initial
temperature ratio for internal thermalizations, $\xi_i=T_{\rm DM,i}/T_{\rm
SM,i}$. This scenario necessarily implies inflaton-mediated scatterings between
DM and SM after reheating, with a rate fixed by the relic abundance of DM and
the reheat temperature. These scatterings can be important for an inflaton mass
and reheat temperature as high as $\mathcal{O}(10^7 {~\rm GeV})$ and
$\mathcal{O}(10^9{~\rm GeV})$, respectively, since the thermally averaged
collision terms become approximately independent of the inflaton mass when the
bath temperature is larger than the mass. The impact of these scatterings on DM
cosmology is studied modeling the perturbative reheating physics by a
gauge-invariant set of inflaton interactions upto dimension-5 with the SM gauge
bosons, fermions and the Higgs fields. It is observed that an initially lower
(higher) DM temperature will rapidly increase (decrease), even with very small
couplings to the inflaton. There is a sharp lower bound on the DM mass below
which the relic abundance cannot be satisfied due to faster back-scatterings
depleting DM quanta to SM particles. For low DM masses, the CMB constraints
become stronger due to the collisions for $\xi_i<1$, probing values as small as
$\mathcal{O}(10^{-4})$, and weaker for $\xi_i>1$. The BBN constraints become
stronger due to the collisions for lower DM masses, probing $\xi_i$ as small as
$\mathcal{O}(0.1)$, and weaker for higher DM mass. Thus inflaton-mediated
collisions with predictable rates, relevant even for high-scale inflation
models, can significantly impact the cosmology of light DM.
