  We study the evolution of the global stellar mass density in a Lambda cold
dark matter universe using two different types of hydrodynamical simulations
(Eulerian TVD and SPH) and the analytical model of Hernquist & Springel (2003).
We find that the theoretical calculations all predict both a higher stellar
mass density at z~3 than indicated by current observations, and that the peak
of the cosmic star formation rate history should lie at z~5. Such a star
formation history implies that as much as (70%, 30%) of the total stellar mass
density today must already have formed by z=(1, 3). Our results suggest that
current observations at z~3 are missing as much as 50% of the total stellar
mass density in the Universe, perhaps owing to an inadequate allowance for dust
obscuration in star-forming galaxies, limited sample sizes, or cosmic variance.
We also compare our results with some of the updated semi-analytic models of
galaxy formation.
