Cosmic Optical Background Excess, Dark Matter, and Line-Intensity Mapping

Recent studies using New Horizons's Long Range Reconnaisance Imager (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 2. We investigate whether this excess, detected at ∼4σ significance, is due to axionlike dark matter that decays to monoenergetic photons. We compute the spectral energy distribution from such decays and the contribution to the flux measured by LORRI. Assuming that axionlike particles make up all of the dark matter, the parameter space unconstrained to date that explains the measured excess spans masses and effective axion-photon couplings of 8-20 eV masses and 3-6×10^{-11}  GeV^{-1}, respectively. If the excess arises from dark-matter decay to a photon line, there will be a significant signal in forthcoming line-intensity mapping measurements that will allow the discrimination of this hypothesis from other candidates.