Though the range of viable dark matter (DM) masses spans 90 orders of magnitude, the natural scenario where its abundance is set by thermal contact with ordinary matter in the early universe significantly restricts this window to ~MeV to multi-TeV masses. This thermal origin implies an interaction between DM and Standard Model (SM) particles, and thus the rich possibility that it may be produced via accelerated beams.

Despite a broad program of LHC searches for DM candidates above the GeV scale, important blind spots remain for beyond-the-SM particles with compressed mass spectra: a generic feature of many of the simplest DM models. I will discuss how the CMS Experiment is enabling new searches in this space through the reconstruction of ultra-low-energy leptons, and how new paradigms for real-time data analysis will open even greater possibilities for Dark Sector searches at the HL-LHC. Lastly I will introduce the capabilities and plans of the Light Dark Matter eXperiment (LDMX), a electron beam fixed-target setup at SLAC, that will test key benchmarks of sub-GeV DM by leveraging LHC technologies.