  A scalar particle with a relic density set by annihilations through a Higgs
portal operator is a simple and minimal possibility for dark matter. However,
assuming a thermal cosmological history this model is ruled out over most of
parameter space by collider and direct detection constraints. We show that in
theories with a non-thermal cosmological history Higgs portal dark matter is
viable for a wide range of dark matter masses and values of the portal
coupling, evading existing limits. In particular, we focus on the string theory
motivated scenario of a period of late time matter domination due to a light
modulus with a decay rate that is suppressed by the Planck scale. Dark matter
with a mass < GeV is possible without additional hidden sector states, and this
can have astrophysically relevant self-interactions. We also study the
signatures of such models at future direct, indirect, and collider experiments.
Searches for invisible Higgs decays at the high luminosity LHC or an e+ e-
collider could cover a significant proportion of the parameter space for low
mass dark matter, and future direct detection experiments will play a
complementary role.
