  Dedicated underground experiments searching for dark matter have little
sensitivity to GeV and sub-GeV masses of dark matter particles. We show that
the decay of B mesons to K(K^*) and missing energy in the final state can be an
efficient probe of dark matter models in this mass range. We analyze the
minimal scalar dark matter model to show that the width of the decay mode with
two dark matter scalars B\to KSS may exceed the decay width in the Standard
Model channel, B\to K\nu\bar\nu, by up to two orders of magnitude. Existing
data from B physics experiments almost entirely exclude dark matter scalars
with masses less than 1 GeV. Expected data from B factories will probe the
range of dark matter masses up to 2 GeV.
