Well your statement was: > As a sanity check, the fact that no other intelligent life has been observed is evidence that it is unlikely. Plus some claims abou

Well your statement was: > As a sanity check, the fact that no other intelligent life has been observed is evidence that it is unlikely. Plus some claims about the uniqueness of our solar system which are not necessarily true, given the small sampling of planet's we've found. There is so much bias in what we observe that I don't think you can make those statements, whether or not you're implying that a species would be galaxy-conquering. Given these biases I think the odds may well be much better than 100 billion - though I'm not current enough to judge whether that's an appropriate estimate based on curent astrophysics. There's so much uncertainty that it could be orders of magnitude different in either direction.