  We present predictions of the correlation between the Lyman-alpha forest
absorption in quasar spectra and the mass within \sim 5 Mpc/h (comoving) of the
line of sight, using fully hydrodynamic and hydro-PM numerical simulations of
the cold dark matter model supported by present observations. The observed
correlation based on galaxies and the Lya forest can be directly compared to
our theoretical results, assuming that galaxies are linearly biased on large
scales. Specifically, we predict the average value of the mass fluctuation,
<delta_m>, conditioned to a fixed value of the Lya forest transmitted flux
delta_F, after they have been smoothed over a 10 Mpc/h cube and line of sight
interval, respectively. We find that <delta_m>/sigma_m as a function of
delta_F/sigma_F has a slope of 0.6 at this smoothing scale, where sigma_m and
sigma_F are the rms dispersions (this slope should decrease with the smoothing
scale). We show that this value is largely insensitive to the cosmological
model and other Lya forest parameters. Comparison of our predictions to
observations should provide a fundamental test of our ideas on the nature of
the Lya forest and the distribution of galaxies, and can yield a measurement of
the bias factor of any type of galaxies that are observed in the vicinity of
Lya forest lines of sight.
