  Here we present world-leading sensitivity to light ($< 170$ MeV) dark matter
(DM) using beam-dump experiments. Dark sector particles produced during pion
decay at accelerator beam-dumps can be detected via scattering in neutrino
detectors. The decay of nuclei excited by the inelastic scattering of DM is an
unexploited channel which has significantly better sensitivity than similar
searches using the elastic scattering channel. We show that this channel is a
powerful probe of DM by demonstrating sensitivity to the thermal relic
abundance benchmark in a scalar DM dark-photon portal model. This is achieved
through the use of existing data, obtained by the KARMEN experiment over two
decades ago, which allow us to set world-leading constraints on this model over
a wide mass range. With experimental improvements planned for the future, this
technique will be able to probe the thermal relic benchmark for fermionic DM
across a wide mass range.
