  In this talk, I explained how the observed dark matter (DM) relic abundance
can be accounted for in models composed of three sectors (the DM, the Standard
Model (SM) and a light mediator) connected to each other. This scenario is
explored in the context of the massive dark photon model in which the DM is
connected to the SM through the kinetic mixing between the hypercharge gauge
boson and the vector gauge boson associated to a new $U(1)'$ gauge symmetry. In
such portal models, the DM relic abundance can be produced in no less than nine
regimes along five dynamical mechanisms. In particular, I emphasize the
sequential freeze-in dynamical mechanism which can occurs when the dark photon
is massive and consists in two successive freeze-in mechanisms.
