A Spectroscopic Search for Optical Emission Lines from Dark Matter Decay

We search for narrow-line optical emission from dark matter decay by stacking dark-sky spectra from the Dark Energy Spectroscopic Instrument (DESI) at the redshift of nearby galaxies from DESI’s Bright Galaxy and Luminous Red Galaxy samples. Our search uses regions separated by 5-20 arcsec from the centers of the galaxies, corresponding to an impact parameter of approximately 50 kpc. No unidentified spectral line shows up in the search, and we place a flux limit of 10^-19ergs/s/cm²/arcsec² on emissions in the optical band (3000≲λ≲9000 Å), which corresponds to 34 in AB magnitude in a normal broadband detection. This detection limit is also three orders of magnitude lower than the measured extragalactic background light (EBL), which rules out the possibility that narrow line emissions in optical from dark matter decay is a major source of the EBL.