We study the direct detection prospects for a representative set of
simplified models of sub-GeV dark matter (DM), accounting for existing
terrestrial, astrophysical and cosmological constraints. We focus on dark
matter lighter than an MeV, where these constraints are most stringent, and
find three scenarios with accessible direct detection cross sections: (i) DM
interacting via an ultralight kinetically mixed dark photon, (ii) a DM
sub-component interacting with nucleons or electrons through a light scalar or
vector mediator, and (iii) DM coupled with nucleons via a mediator heavier than
~ 100 keV.Comment: 44 pages, 13 figures, reference added and minor updates to some of
the constraints, conclusions unchange
