We consider the sensitivity of high luminosity neutrino experiments to light stable states, as arise in scenarios of MeV-scale dark matter. To ensure the correct thermal relic abundance, such states must annihilate to the Standard model via light mediators, providing a portal for access to the dark matter state in colliders or fixed targets. This framework implies that neutrino beams produced at a fixed target will also carry an additional “dark matter beam”, which can mimic neutrino scattering off electrons or nuclei in the detector. We therefore develop a Monte Carlo code to simulate the production of a dark matter beam at two proton fixed-target facilities with high luminosity, LSND and MiniBooNE, and with this simulation determine the existing limits on light dark matter. We find in particular that MeV-scale dark matter scenarios motivated by an explanation of the galactic 511 keV line are strongly constrained. / Graduate