  The comparison of cosmic abundances of the light elements with the density of
baryonic stars and gas in the universe today provides a critical test of big
bang theory and a powerful probe of the nature of dark matter. A new technique
allows determination of cosmic deuterium abundances in quasar absorption clouds
at large redshift, allowing a new test of big bang homogeneity in diverse, very
distant systems. The first results of these studies are summarized, along with
their implications. The quasar data are confronted with the apparently
contradictory story from the helium-3 abundances measured in our Galaxy. The
density of baryonic stars and gas in the universe today is reviewed and
compared with the big bang prediction.
