  Numerical simulations of standard cosmological scenarios have now reached the
degree of sophistication required to provide tentative answers to the
fundamental question: Where and when were the heavy elements formed? Averaging
globally, these simulations give a metallicity that increases from 1% of the
solar value at $z=3$ to 20% at present. This conclusion is, in fact,
misleading, as it masks the very strong dependency of metallicity on local
density. At every epoch higher density regions have much higher metallicity
than lower density regions. Moreover, the highest density regions quickly
approach near solar metallicity and then saturate, while more typical regions
slowly catch up. These results are much more consistent with observational data
than the simpler picture (adopted by many) of gradual, quasi-uniform increase
of metallicity with time.
