  Detection of electron recoils by dark matter (DM) may reveal the structure of
the dark sector. We consider a scenario where a heavier DM particle
inelastically scatters off an electron and is converted into a lighter DM
particle. A small mass difference between the two DM particles is transferred
into electron recoil energy. We investigate the DM-electron interaction
mediated by a massive dark photon and evaluate the inelastic DM scattering
rate, taking account of the atomic structure. It is found that the scattering
rate is significantly enhanced because of the small mass splitting, which
allows for a small momentum transfer matched with the size of the electron wave
function. We show that there exists a viable parameter space which explains the
excess of electron recoil events around 2-3 keV recently reported by the
XENON1T experiment.
