The cosmic optical background excess, line-intensity mapping

<!--HTML--><p>Recently, the New Horizons spacecraft was able to detect, for the first time, the cosmic background of optical light. &nbsp;The signal was twice that expected from galaxy number counts. &nbsp;This excess can be explained in terms of two-photon decay of a dark-matter axion, a hypothesis that will be tested with line-intensity mapping very high significance with the impending launch of NASA’s SphereX. &nbsp;I will discuss line-intensity mapping and show how it can be used to probe dark matter, neutrino decays, and other exotic physics.</p>