The cosmic optical background excess, dark matter, and line-intensity
  mapping
  Recent studies using New Horizons LORRI images have returned the most precise
measurement of the cosmic optical background to date, yielding a flux that
exceeds that expected from deep galaxy counts by roughly a factor of two. We
investigate whether this excess, detected at $\sim 4\sigma$ significance, is
due to dark matter that decays to a monoenergetic photon with a rest-frame
energy in the range $0.5-10$ eV. We derive the spectral energy distribution
from such decays and the contribution to the flux measured by LORRI. The
parameter space that explains the measured excess with decays to photons with
energies $E\gtrsim 4$ eV is unconstrained to date. If the excess arises from
dark-matter decay to a photon line, there will be a significant signal in
forthcoming line-intensity mapping measurements. Moreover, the ultraviolet
instrument aboard New Horizons (which will have better sensitivity and probe a
different range of the spectrum) and future studies of very high-energy
$\gamma$-ray attenuation will also test this hypothesis and expand the search
for dark matter to a wider range of frequencies.
