Recently, the New Horizons spacecraft was able to detect, for the first time, the cosmic background of optical light.  The signal was twice that expected from galaxy number counts.  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.  I will discuss line-intensity mapping and show how it can be used to probe dark matter, neutrino decays, and other exotic physics.