Are there millions of other civilizations, are we quite rare, or even unique?
In our Galaxy there are 300 billion stars. 300,000,000,000.
If we are unique that is a 300 billion to one chance. Like me hiding a dinner plate in Wales, (or Israel or Slovenia) and you guessing exactly where it is. To the inch. Improbable enough to be dismissed.
So why have we not heard from another civilisation? These may be possible filters.
Given the high probability of there being loads of civilisations in our galaxy (let alone the billions of other galaxies), The chances that all civilizations would have destroyed themselves, or have not, at least occasionally, made contact with one another, or lasted long enough to send a reasonably powerfull and unabigous signal for us to recieve are remote enough to be ignored. This leads to what I consider being far and away the most likely scenario:
Think about it.. We are learning that it is a bad thing to disturb a pristine environment. I would like to think that were we to discover 'The Land That Time Forgot', the last thing we would do would be to allow rats, aids, religion, smallpox and alcohol loose on the place along with unrestrained trips by bachelor parties eager to suck the entrails out of the creatures that inhabited it.
If we ever achieve the technology to emigrate from the Earth and sustain a population away from it it is certain that the wishful thinking on my part would have become reality. Let's just imagine we discovered a civilization similar to ours, just a few years behind. Would we really give nuclear bombs and anthrax to Genghis Khan, Henry VIII or Hitler? If we ever achieve the technology to emigrate from the Earth and sustain a population away from it I would postulate we would have developed sufficient 'cloaking' technology to conceal ourselves from emerging civilisations at the same stage of development as we are now.
Would we observe? Probably.
Would we allow them to fight to the death? Definitely, but perhaps preserving a few samples or, given a sufficient chance of total destruction of a planet, even remove the species capable of such destruction to preserve the remaining bio-diversity.
Would we reveal ourselves? Not until such a time that it became impossible to stay hidden.
What would happen when it was no longer possible to stay hidden, when that discovered civilization discovered our planet and the tell tale signs of life and industry? We would say "Hi, well done, you've survived, welcome to the club.. Now fucking hide from these little planets here and here, they are in crux mode. We like your music!"
It is critical to think about stages of technological development. Any creatures malevolent enough to go on a rampage of crime and conquest throughout space would have destroyed themselves long, long before they acquire the technology to do so. Observing our tenuous hold on this small blue dot where we live and our treatment of it and each other, it is not difficult to imagine this happens very frequently.
If we want our DNA to survive (and we do!) we must accept this, and embrace all actions to reduce risk of destruction and increase the chances of sustainable procreation. Then we will discover the true wonder of life and how our paltry 8.7 million species make finding that plate in Wales look like good odds.