  We study scalar dark matter production and reheating via renormalizable
inflaton couplings, which include both quartic and trilinear interactions.
These processes often depend crucially on collective effects such as
resonances, backreaction and rescattering of the produced particles. To take
them into account, we perform lattice simulations and map out parameter space
producing the correct (non-thermal) dark matter density. We find that the
inflaton-dark matter system can reach a quasi-equilibrium state during
preheating already at very small couplings, in which case the dark matter
abundance becomes independent of the inflaton-dark matter coupling and is
described by a universal formula. Dark matter is readily overproduced and even
tiny values of the direct inflaton couplings can be sufficient to get the right
composition of the Universe, which reaffirms their importance in cosmology.
