We show that spatial correlations in a stochastic large scale velocity field in an otherwise smooth intergalactic medium ( homogeneous comoving density ) superposed on the general Hubble flow , may cause a ‘ line-like ’ structure in QSO spectra similar to the population of unsaturated Ly \alpha forest lines which usually are attributed to individual clouds with 10 ^ { 11 } \la N _ { HI } \la 5 \times 10 ^ { 13 } cm ^ { -2 } .
Therefore there is no clear observational distinction between a diffuse intergalactic medium and discrete intergalactic clouds .
It follows that the HI-density in the diffuse intergalactic medium might be substantially underestimated if it is determined from the observed intensity distribution near the apparent continuum in high resolution spectra of QSOs .
Our tentative estimate implies a diffuse neutral hydrogen opacity \tau _ { GP } \sim 0.3 at z \sim 3 and a current baryon density \Omega _ { IGM } \simeq 0.08 , assuming a Hubble constant { H } _ { 0 } = 70 km s ^ { -1 } Mpc ^ { -1 } .