Why are there no signs of aliens, given the vastness of the galaxy and the apparent abundance of opportunities for life to develop? This is the Fermi paradox. Here’s a possible solution to it. I’ve not seen it proposed before, though I don’t know the literature well.
Suppose the following claims are true:
1) There is a multiverse. (It may not be strictly necessary to assume this, but it makes exposition easier.)
2) Any universe that is capable of supporting life is also fragile, in the sense that a relatively small, highly localized ‘pinprick’ event can cause the whole thing to collapse, as a pinprick can destroy an inflated balloon.
3) ‘Pinprick’ events do not occur naturally. (Again, it may not be strictly necessary to assume this.)
4) An advanced technological civilization will almost certainly create a pinprick event inadvertently as soon as it is capable of doing so. (Suppose that there is a technological process that promises huge benefits in say, in cheap energy or interstellar travel, but which will also, and unforeseeably, create a pinprick event.)
Then we have a solution to the Fermi paradox. Every universe in which a technological civilization develops is rapidly destroyed by that civilization — or by another that develops around the same time. No universe contains more than a very small number of technological civilizations, and we shouldn’t expect to see signs of another one in our galaxy.
And we’re going to destroy the universe soon.