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EmergentSubject2336 t1_jab2cf9 wrote

>Surely we would have detected such huge technological structures, communications technologies, etc.,

Exactly, the fact we don't see any of that implies they aren't around yet in our past lightcone( I don't claim in the entire universe, only as far as we can see). And, the idea is that we wouldn't have emerged if the universe was already filled with such life.

The selection effect (anthropic principle) here is that the only point in time where a civilization like ours could emerge is when the universe hasn't yet been filled with transformative life. And we are normal (Copernican principle) in that all young civilizations like us likewise observe an empty universe, since otherwise they wouldn't be there. But the emptiness will go away pretty soon.

You probably meant was that aliens are quiet: Aliens that don't expand won't affect anything and get steamrolled. They may as well not be there. What counts is that at least some do expand as the other commenter pointed out.

There is a whole framework around this to model this called Grabby Aliens. You can read and watch more about that here: https://grabbyaliens.com

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