  The Gunn-Peterson effect, absorption of Lya photons by a homogeneous
component of the intergalactic medium (IGM), potentially provides a test of Big
Bang Nucleosynthesis (BBN). With a lower limit on the UV radiation field
estimated from the contribution due to QSOs, a measurement of the Lya opacity
of the intergalactic medium would permit the derivation of a lower bound to the
baryonic density of the universe. The effect, however, has continually eluded a
convincing detection, both in HI and HeII, despite extensive searches. Recent
cosmological hydrodynamical simulations of structure formation in the
intergalactic medium suggest an explanation for its absence. In a Cold Dark
Matter dominated cosmology, the fragmentation of the baryons is nearly
complete, leaving a negligible remnant to comprise a smoothly distributed
component. The fragmentation extends even into regions that are underdense,
where it gives rise to most of the optically thin HI systems and nearly all of
the HeII systems, both thin and saturated. The result is a Lya opacity from a
smooth IGM that is suppressed by over two orders of magnitude from the BBN
value.
