We have been considering the Great Filter at Orion's Arm for quite some time. Bostrom has the right ideas; there are a number of critical periods in the development of a civilisation which might filter out most, or all, potential contenders. In order for our fictional scenario to work, a scenario which has several advanced civilisations in each galaxy, we have had to imagine a Great Filter which somehow removes star-faring civilisations from the Galaxy
after they have expanded for some limited period, but before they colonise the entire galaxy and set up homesteads on every rock, nook, and cranny.
One possibility is that an advanced civilisation always collapses due to warfare: internal strife, or in conflict with other nerby civilisations. Or there may be an existential threat from high technology itself- advanced automatic defences might decide to wipe out all potential enemies, or some experimental technology might backfire and destroy the civilisation in some way. But I suspect we would see evidence of this in distant galaxies. For instance an advanced civilisation might decide to build a large number of superweapons, such as the so-called Nicoll-Dyson Laser
http://en.wikipedia.org/wiki/James_D...ll-Dyson_Laser
which could wipe out a solar system in a single blow. If an advanced civilisation had too many such weapons they might run the risk of an accidental war. Because of light speed delays all the potential targets could be destroyed by beams emitted by lasers which are subsequently destroyed themselves.
But if such energetic warfare occurs frequently we would surely see the evidence in distant galaxies. Perhaps that is the explanation for the anomalously bright gamma-ray burst seen recently...
Another possibility is that very advanced civilisations remove themselves from our universe and create artificial universes of their own; some cosmologists think that it may be possible to create new universes in the laboratory, and perhaps tune them to be more suitable for intelligent processes. If you can make your own paradise, why stay in this cosmos where stars and planets are so far apart? When I am feeling hopeful, I tend to favour this explanation. Perhaps these advanced civilisations in their basement universes have a way of communicating with our own world when necessary; perhaps not. A daunting thought.
But most of the time I tend towards thinking that civilisations are very rare, and the Filter is in our past.