  The identity of dark matter is being sought with increasingly sensitive and
voluminous underground detectors. Recently the XENON1T collaboration reported
excess electronic recoil events, with most of these having recoil energies
around $1-30$ keV. We show that a straightforward model of inelastic dark
matter produced via early universe thermal freeze-out annihilation can account
for the XENON1T excess. Remarkably, this dark matter model consists of a few
simple elements: sub-GeV mass Dirac fermion dark matter coupled to a lighter
dark photon kinetically mixed with the Standard Model photon. A scalar field
charged under the dark U(1) gauge symmetry can provide a mass for the dark
photon and splits the Dirac fermion component state masses by a few keV, which
survive in equal abundance and interact inelastically with electrons and
nuclei.
