  We calculate the post-recombination contribution to the Cosmic Microwave
Background (CMB) spectral distortion due to general exotic energy injections,
including dark matter (DM) decaying or annihilating to Standard Model
particles. Upon subtracting the background distortion that would be present
even without such energy injections, we find residual distortions that are
still potentially large enough to be detectable by future experiments such as
PIXIE. The distortions also have a high-energy spectral feature that is a
unique signature of the injection of high-energy particles. We present a
calculation of the global ionization history in the presence of decaying dark
matter with sub-keV masses, and also show that previous calculations of the
global ionization history in the presence of energy injection are not
significantly modified by these additional spectral distortions. Our improved
treatment of low-energy electrons allows us to extend calculations of the CMB
anisotropy constraints for decaying DM down to arbitrarily low masses. We also
recast these bounds as constraints on the coupling of axion-like particles to
photons.
