Here's my theory of life in the universe.
First of all a large part is uninhabited because conditions are just too extreme for life to hang around long enough, be it gamma rays, overly active stars, supernovas, some deficiency in the planet, too many extinction level events etc.
Next are the planets where conditions are harsh, but there is some life, mostly very simple creatures that are very tough and resilient, but they are too busy merely surviving and there are too few opportunities for natural selection to create enough variation.
Next are the planets that have easier conditions and more variation, but it's still mostly pond snot, algae, fungi, plants, trees, worms, dinosaurs, bugs, ponies etc
And then you get to creatures that are quite intelligent and advanced, but are the "wrong kind" of intelligence like chimps, dolphins, elbonians ... In the whole history of our planet only a single species created technology you would notice on another planet. And then there is the extinction problem, imagine you have the right planet, the right intelligent species and they reach a technology level similar to the late 19th century, radio technology is only a few years away and then there is an extinction event ... the universe is so big it probably happened several times ...
It could be that life is very common, but proper intelligent life is rare and even if there are many intelligent aliens out there we might notice it could be that we are "in between" signals, that the various alien races that we might pick up are either too far or stopped emitting for one reason or another, we could be stuck in radio silence for quite a while.
The often mentioned Von Neuman probes might have passed by earth when Homo Erectus barely discovered how to make pointy sticks.
And maybe when we finally get a signal it turns out they are so radically different from us we might not be able to carry the most basic of conversations.
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