  We investigate Hawking evaporation of a population of primordial black holes
(PBHs) prior to Big Bang Nucleosynthesis (BBN) as a mechanism to achieve
asymmetric reheating of two sectors coupled solely by gravity. While the
visible sector is reheated by the inflaton or a modulus, the dark sector is
reheated by PBHs. Compared to inflationary or modular reheating of both
sectors, there are two advantages: $(i)$ inflaton or moduli mediated operators
that can subsequently thermalize the dark sector with the visible sector are
not relevant to the asymmetric reheating process; $(ii)$ the mass and abundance
of the PBHs provide parametric control of the thermal history of the dark
sector, and in particular the ratio of the temperatures of the two sectors.
Asymmetric reheating with PBHs turns out to have a particularly rich dark
sector phenomenology, which we explore using a single self-interacting real
scalar field in the dark sector as a template. Four thermal histories,
involving non-relativistic and relativistic dark matter (DM) at chemical
equilibrium, followed by the presence or absence of cannibalism, are explored.
These histories are then constrained by the observed relic abundance in the
current Universe and the Bullet Cluster. The case where PBHs dominate the
energy density of the Universe, and reheat both the visible as well as the dark
sectors, is also treated in detail.
