I can only use our own communicating, intelligent civilization as an example of the sort of proof I am looking for. And it might be a poor exemplar at that. But I find it hard to believe that any other intelligences exist since some evidence of another civilization hasn't yet floated by, been received via radio wave, or even been carved on a rock somewhere yet. Something as majestic as a wrecked spacecraft or as trivial as an empty Betelgeusean soda pop bottle or the classic "take me to your leader" broadcast sought for so long by SETI would be all it would take. The fact that no such detritus has yet been found makes me wonder if such "evidence" will ever be found because it simply doesn't exist.
it isn't that surprising that even if their are other intragalatic intelligences we haven't found them yet and the distances to other galaxies are so large that our methods of search simply will not find them if they exist.
Intelligence is curious. We would have seen or heard something by now (either through the fruit of our active exploration or simply by accident) if there were anything out there to hear or see.
But when you consider the percentage of the galaxy we've explored, whether it be examining rocks or radio waves, we've explored almost none of it. If there were another civilization with a radio luminosity just like Earth's, how close do you think it would have to be for us to see it? If there were a ten thousand alien soda pop bottles on the moon, how many Apollo missions would it take to find one?
I think you undercut how vast the universe is.
Within our galaxy alone there are roughly 200,000,000,000 stars.
If an advanced civilization visited 10 stars a year, it would still take them twenty billion years to see them all, which is (probably) older then the age of the universe.
My deepest apologies for not posting much here. I have been terribly busy. I will be able to post tonight for sure!
Sincerely,
RA