This is my idea in response to Fermi’s question Where Are They? to speculations that the Galaxy must be teaming with not only life but intelligent civilizations. So what could cause so many intelligent civilizations to be missing or to go extinct? Self destruction via war?, pollution and environmental self destruction? or the fact the the universe has many high energy event that would sterilize a volume of surrounding space. These volumes devoid of life are the empty volumes like the hollow spaces in a block of Swiss Cheese so hence the idea of a Swiss Cheese Galaxy. Since we can list the kinds of high energy events and calculate the volume they would sterilize based on the distance is would take this energy to dissipate to a survivable level this is a tractable problem for astrophysics.
In other words we know enough to test the Swiss Cheese galaxy theory in a statistical sense from knowing the various kind of high energy events, the chronology of stellar evolution that result in these high energy events, stellar densities from sky surveys, and the ability to calculate the distances of surrounds space where life would be extinguished. I don’t have the knowledge or calculation abilities to do these calculations but at least I have a theory that might answer Fermi’s question Where are they? or the related question what might have happened to them.
Since the Galactic Center has the greatest stellar density and the greatest probability of associated high energy events the level of sterilization there must are greater than in the lower stellar density of the galactic arms or the even less dense stellar population in the space between the galactic arms. The Earth being in a galactic arm may simply have been lucky in escaping a nearby high energy event which could cause the extinction of terrestrial life!
So this is Dave’s conjecture about one factor in the Drake Equation. It remains for much better qualified people to undertake the statistical calculations involved to see if this conjecture has some merits in a statistical universe.