The diffuse cosmic background at ultraviolet, optical, and infrared wavelengths is produced by the emission of all stars across the history of the Universe. Its knowledge would thus inform us about stellar activity, galaxy evolution, dust extinction and the expansion rate of the Universe. In this talk I will report on an unprecedented measurement of this background using NASA’s Fermi mission and how it has allowed us to measure the history of light in the Universe and the expansion rate of the Universe at recent times.
