Telescope search for a 3-eV to 8-eV axion

Axions of mass 3--8 eV should have a cosmological abundance of about 50 cm^-3 and reside in rich clusters of galaxies. Their decays to two photons will produce a line at a wavelength λₐ{~}3100--8300 {} (width {Δ}{λ}/{λ}{~}10^-2). We have searched unsuccessfully for such a feature in the intergalactic light of three rich clusters, closing this ``window,'' and leaving open only the window from 10^-6 to 10^-3 eV. This implies that if the axion exists, it likely comprises the dark matter. Our flux limits are of relevance to other relics whose decays produce monoenergetic photons.