  We study postinflationary scalar dark matter production via its non-minimal
coupling to gravity. During the inflaton oscillation epoch, dark matter is
produced resonantly for a sufficiently large non-minimal coupling $\xi\gtrsim
5$. We find that backreaction on the curvature and rescattering effects
typically become important for the values of $\xi$ above $30$, which invalidate
simple estimates of the production efficiency. At large couplings, the dark
matter yield becomes almost independent of $\xi$, signifying approximate
quasi-equilibrium in the inflaton-dark matter system. Although the analysis
gets complicated by the presence of apparent negative energy in the Jordan
frame, this behaviour can be regularized by introducing mild dark matter
self-interaction. Using lattice simulations, we delineate parameter space
leading to the correct dark matter relic abundance.
