Maybe aliens have been here before, but it seems unlikely that they've been here during recorded history (~5000 years). And such a theory must go hand-in-hand with another to explain why we haven't detected any signs of any advanced civilizations.
TBH I think the most likely theories are that we are among the most advanced life in the universe, that life is unique or relatively unique and extremely rare (possibly tying in to theory 1), or that other life is so different to what we are used to that we wouldn't know it if we saw it (and possibly the other way round).
My favourite (albeit scary) theory is that the an early, highly advanced civilization wiped out all other advanced civilizations. It continues to seek out and destroy other intelligent life before it has a chance to spread to other planets. That would mean our time is potentially quite short before we're wiped out.
TBH I think the most likely theories are that we are among the most advanced life in the universe, that life is unique or relatively unique and extremely rare (possibly tying in to theory 1), or that other life is so different to what we are used to that we wouldn't know it if we saw it (and possibly the other way round).
My favourite (albeit scary) theory is that the an early, highly advanced civilization wiped out all other advanced civilizations. It continues to seek out and destroy other intelligent life before it has a chance to spread to other planets. That would mean our time is potentially quite short before we're wiped out.
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