  If cosmic inflation was driven by an electrically neutral scalar field stable
on cosmological time scales, the field necessarily constitutes all or part of
dark matter (DM). We study this possibility in a scenario where the inflaton
field $s$ resides in a hidden sector, which is coupled to the Standard Model
sector through the Higgs portal $\lambda_{hs}
s^2\mathcal{H}^\dagger\mathcal{H}$ and non-minimally to gravity via $\xi_s s^2
R$. We study scenarios where the field $s$ first drives inflation, then reheats
the Universe, and later constitutes all DM. We consider two benchmark scenarios
where the DM abundance is generated either by production during reheating or
via non-thermal freeze-in. In both cases, we take into account all production
channels relevant for DM in the mass range from keV to PeV scale. On the
inflationary side, we compare the dynamics and the relevant observables in two
different but well-motivated theories of gravity (metric and Palatini), discuss
multifield effects in case both fields ($s$ and $h$) were dynamical during
inflation, and take into account the non-perturbative nature of particle
production during reheating. We find that, depending on the initial conditions
for inflation, couplings and the DM mass, the scenario works well especially
for large DM masses, $10^2$ GeV$\lesssim m_{s}\lesssim 10^6$ GeV, although
there are also small observationally allowed windows at the keV and MeV scales.
We discuss how the model can be tested through astrophysical observations.
