  We introduce a simple model for the formation of voids. In this model the
underdensity of galaxies in voids is the product of two factors. The first
arises from a gravitational expansion of the negative density perturbation. The
second is due to biasing: galaxies are less likely to form in an underdense
region. One feature of the model is an upper cutoff in void sizes. We calculate
the volume filling factor of characteristic voids for different CDM models and
find that our formation model points to $\Lambda$CDM models as preferred models
of the power spectrum. A natural consequence of our model is that the
underdensity of the dark matter inside voids is smaller than the galaxy
underdensity.
