  We show for the first time that the loop-driven kinetic mixing between
visible and dark Abelian gauge bosons can facilitate dark matter production in
the early Universe by creating a 'dynamic' portal, which depends on the energy
of the process. The required smallness of the strength of the portal
interaction, suited for freeze-in, is justified by a suppression arising from
the mass of a heavy vector-like fermion. The strong temperature sensitivity
associated with the interaction is responsible for most of the dark matter
production during the early stages of reheating.
