here's also the religious explanation which has been missed out. Whatever your prejudices on this subject, for objectivity's sake, this one should have been included.
Pope John Paul II (died 2005) canonised as a Christian saint a carrot peeler nun (ie, very humble, low-level nun) who claimed in her diary that Jesus had told her that the universe is teeming with life. St Faustina.
The Catholic position is that there is no dogma on this matter, although the above makes it an informal teaching. But it's particularly interesting since a Catholic priest invented the Big Bang theory, Lemaitre, and a Catholic monk, Mendel, is considered the father of modern genetics. (Catholicism has never had a dogma of a literal interpretation of the Bible.)
Meanwhile, the ex-atheist, near-death-experience, Arts professor, Howard Storm (now a methodist), says that he was shown by God that not only is the universe teeming with life, but also intelligent life, and that we are either the only or one of very few that rebelled against God. All other planets with intelligent life receive God's blessings without resistence. Considering God's blessings to us include death, I suppose that's not too surprising. :)
The interesting detail in Storm's testimony is why outside life isn't communicating with us (radio waves etc): there is no need as, excepting humans, intelligent beings communicate without need of material means, particularly through 'union' in which they experience each other's existence.
Howard Storm, talking to some old ladies ina Church:
No, it shouldn't be included. This is a scientific discussion. Religion is for theologians and philosophers. Take it there.
Lol!! You're presumably an atheist. Only an atheist prejudice would make such a claim. The question isn't purely scientific. And no reasonable scientist would exclude this possibility. An atheist scientist would, however.
And while there has been an amazing loss of faith among the young, (mostly due to successful misinformation from atheists such as Dawkins), atheism continues to be a minority belief. Most people are at most agnostic, making no atheistic presumptions. And atheism is presumptuous since there has never been a proof of no god, and plenty of evidence that that might be one, starting with the fact that there are so many people claiming to have had personal contact with a god. Which includes myself.