  Motivated by the recent excess in the electron recoil from XENON1T
experiment, we consider the possibility of exothermic dark matter, which is
composed of two states with mass splitting. The heavier state down-scatters off
the electron into the lighter state, making an appropriate recoil energy
required for the Xenon excess even for the standard Maxwellian velocity
distribution of dark matter. Accordingly, we determine the mass difference
between two component states of dark matter to the peak electron recoil energy
at about $2.5\,{\rm keV}$ up to the detector resolution, accounting for the
recoil events over $E_R=2-3\,{\rm keV}$, which are most significant. We include
the effects of the phase-space enhancement and the atomic excitation factor to
calculate the required scattering cross section for the Xenon excess. We
discuss the implications of dark matter interactions in the effective theory
for exothermic dark matter and a massive $Z'$ mediator and provide microscopic
models realizing the required dark matter and electron couplings to $Z'$.
