Landis's Percolation paper
Hi Hank
Geoff Landis makes the reasonable assumption that civilisations might differentiate as they expand into space - thus each new colony is really a separate entity. If each colony, like the home world, is faced with three options (a) extinction (b) stasis (c) expansion, then the spread through the galaxy becomes a probabilistic 'percolation' which is messy enough to leave lots of unoccupied voids throughout the Galaxy.
Here's the paper online.