The observed abundance of lithium in population I stars (young stars)1 seems to have been stable during the past 5 × 109 yr (ref. 2). This stability has been used previously1,3 to estimate the abundance of lithium at the time of formation of the Galaxy and at pre-galactic times1,3. But because the basis for this estimate was not completely satisfactory, we have made new observations, aiming at a better determination of the lithium abundance at the time of formation of the Galaxy. The newly observed stars are extreme population II dwarfs (very old, very metal-poor stars). Their lithium abundance turned out to be significantly lower than the abundance in population I stars. If attributed to the big bang, this lithium abundance suggests a rather low baryonic density of the Universe, which in turn favours, under some assumptions, an open Universe3. Finally, we suggest that the good agreement between the abundance of 7Li and the deuterium 2H abundance3 supports the standard model of the big bang.