I was listening to a podcast (think it might have been the Economist's science and technology one) and they raised this issue when it sames to extra terrestrial intelligent life that never really gets talked about.
We can comprehend that the universe is massive (or maybe we can't be we know it's fucking huge) and given all these moons, planets, stars and galaxies we have to conclude that there is extra terrestrial intelligent life right?
Because while we can just about fathom the scale of the universe we often struggle with the age of it.
If the whole of time from the big bang was a year, wit the big bang being midnight on January 1st then human civilization has been around since a couple of seconds to midnight on December 31st (or about 10,000 years).
So if human civilization continues for another 10,000 years then we'll only exist for a fraction of the universe's time.
If an alien civilization in space lasted for 100,000 years it still wouldn't around for even an hour of the universe's time calendar.
So it is possible that thousands of alien civilizations have existed in the past and will exist in the future but the chances that they could ever overlap with us are tiny given how old the universe is.
The depressing thing is that if SETI or something ever did receive a transmission from an alien civilization it's possible that the sender's entire species, planet or even sun died out millions of years before we ever receive it.