  We investigate the power of low-energy, high-luminosity electron--positron
colliders to probe hidden sectors with a mass below ~10 GeV that couple to
Standard Model particles through a light mediator. Such sectors provide
well-motivated dark matter candidates, and can give rise to distinctive
mono-photon signals at B-factories and similar experiments. We use data from an
existing mono-photon search by BaBar to place new constraints on this class of
models, and give projections for the sensitivity of a similar search at a
future B-factory such as Belle II. We find that the sensitivity of such
searches are more powerful than searches at other collider or fixed-target
facilities for hidden-sector mediators and particles with masses between a few
hundred MeV and 10 GeV. Mediators produced on-shell and decaying invisibly to
hidden-sector particles such as dark matter can be probed particularly well.
Sensitivity to light dark matter produced through an off-shell mediator is more
limited, but may be improved with a better control of backgrounds, allowing
background estimation and a search for kinematic edges. We compare our results
to existing and future direct detection experiments and show that low-energy
colliders provide an indispensable and complementary avenue to search for light
dark matter. The implementation of a mono-photon trigger at Belle II would
provide an unparalleled window into such light hidden sectors.
