  We demonstrate how a dark sector can provide a crucial link between a
vacuum-dominated early universe and the later radiation-dominated epoch. As an
example, we consider a feeble axion-like coupling of dark non-Abelian gauge
fields with a single inflaton. In this scenario, a dark heat bath emerges at
the end of inflation. As the dark sector cools down, its gauge fields confine
into composite glueball states. The reheating of the visible sector is realized
through portal interactions. After confinement, discrete symmetries protect
part of the glueball states that form relic dark matter, while the others keep
decaying into Standard Model degrees of freedom. The dark relic abundance
depends mostly on the departure from equilibrium dynamics after the phase
transition, constraining the confinement scale of the dark sector. Moreover,
indirect detection and Big-Bang nucleosynthesis set bounds on the scale of
portal interactions. We show that in a narrow but non-vanishing region of the
parameter space, the correct dark matter abundance and sufficient reheating
temperature are simultaneously reached.
