Call it the “cosmic fog.” Or the “extragalactic background light.” Astronomers use these names to describe the light that has left every star and now lingers in the universe, including light from stars that have burned out. Studying this background light can be tricky because objects that produce or reflect light — stars, galaxies, dust, bright stuff hurled through space by a black hole — can get in the way. But, astronomers have just determined how super-bright galaxies can be used to detect the faint glow of the cosmic fog.