  We consider the sensitivity of fixed-target neutrino experiments at the
luminosity frontier to light stable states, such as those present in models of
MeV-scale dark matter. To ensure the correct thermal relic abundance, such
states must annihilate via light mediators, which in turn provide an access
portal for direct production in colliders or fixed targets. Indeed, this
framework endows the neutrino beams produced at fixed-target facilities with a
companion `dark matter beam', which may be detected via an excess of elastic
scattering events off electrons or nuclei in the (near-)detector. We study the
high luminosity proton fixed-target experiments at LSND and MiniBooNE, and
determine that the ensuing sensitivity to light dark matter generally surpasses
that of other direct probes. For scenarios with a kinetically-mixed U(1)'
vector mediator of mass m_V, we find that a large volume of parameter space is
excluded for m_DM ~ 1-5 MeV, covering vector masses 2 m_DM < m_V < m_eta and a
range of kinetic mixing parameters reaching as low as kappa ~ 10^{-5}. The
corresponding MeV-scale dark matter scenarios motivated by an explanation of
the galactic 511 keV line are thus strongly constrained.
