  Superconducting transition-edge sensors (TESs), known for their high
single-photon detection efficiency and low background, are increasingly being
used in rare event searches. We present the first comprehensive
characterization of optical TES backgrounds, identifying three event types:
high-energy, electrical noise, and photon-like events. We experimentally verify
and simulate the source of the high-energy events. We develop an algorithm to
isolate photon-like events, the expected signal in dark matter searches,
achieving record-low photon-like dark count rates in the 0.8-3.2 eV energy
range.
