  While second and higher order correlations of the light distribution have
received extensive study, the lowest order probability distribution function
(PDF) -- the probability that a unit volume of space will emit a given amount
of light -- has received very little attention. We estimate this function with
the aid of hydrodynamic simulations of the Lambda CDM model, finding it
significantly different from the mass density PDF, and not simply related to it
by linear bias or any of the other prescriptions commonly adopted. If the
optical light PDF is, in reality, similar to what we find in the simulations,
then some measures of Omega_M based on mass-to-light ratio and the cosmic
virial theorem will have significantly underestimated Omega_M. Basically, the
problem is one of selection bias, with galaxy forming regions being
unrepresentative of the dark matter distribution in a way not described by
linear bias. Knowledge of the optical PDF and the plausible assumption of a
log-normal distribution for the matter PDF will allow one to correct for these
selection biases. We find that this correction (which amounts to 20-30%) brings
the values of Omega_M estimated by using the mass-to-light ratio and the cosmic
virial theorem to the range 0.2-0.3, in better agreement with the WMAP result
than the uncorrected estimates. In addition, the relation between mass and
light PDFs gives us insight concerning the nature of the void phenomenon. In
particular our simulation indicates that 20% of mass is distributed in voids,
which occupy 85% of volume in the universe.
