A Model of Void Formation

We introduce a simple model for the formation of voids. In this model theunderdensity of galaxies in voids is the product of two factors. The firstarises from a gravitational expansion of the negative density perturbation. Thesecond is due to biasing: galaxies are less likely to form in an underdenseregion. One feature of the model is an upper cutoff in void sizes. We calculatethe volume filling factor of characteristic voids for different CDM models andfind that our formation model points to $\Lambda$CDM models as preferred modelsof the power spectrum. A natural consequence of our model is that theunderdensity of the dark matter inside voids is smaller than the galaxyunderdensity.