Measurements of the deuterium abundance in the early Universe provide a sensitive test of the ’standard’ Big Bang cosmology. The probable detection of deuterium absorption by a gas cloud between us and a distant quasar suggests an abundance much greater than estimated from observations in the Milky Way, and consistent with the amount of presently observed luminous matter comprising all the baryons in the Universe. The same spectra imply a cosmic background temperature for the early Universe which is consistent with our expectations from standard Big Bang cosmology.