  We show that the rate for dark matter-electron scattering in an arbitrary
material is determined by an experimentally measurable quantity, the complex
dielectric function, for any dark matter interaction that couples to electron
density. This formulation automatically includes many-body effects, eliminates
all systematic theoretical uncertainties on the electronic wavefunctions, and
allows a direct calibration of the spectrum by electromagnetic probes such as
infrared spectroscopy, X-ray scattering, and electron energy-loss spectroscopy
(EELS). Our formalism applies for several common benchmark models, including
spin-independent interactions through scalar and vector mediators of arbitrary
mass. We discuss the consequences for standard semiconductor and superconductor
targets, and find that the true reach of superconductor detectors for light
mediators exceeds previous estimates by several orders of magnitude, with
further enhancements possible due to the low-energy tail of the plasmon. Using
a heavy-fermion superconductor as an example, we show how our formulation
allows a rapid and systematic investigation of novel electron scattering
targets.
