I liked the Dark Forest theory put forward in, well, The Dark Forest [1]. Advanced Alien civilizations are all across the galaxy, but everyone is keeping quiet and trying to not get noticed as being noticed is a death sentence. Not necessarily because there are evil civilizations hell bent on destruction, but because no one can be sure that the person who has noticed them is not going to destroy them simply because they in turn are not sure whether they know they are there and are hostile, and so on. This uncertainty arises due to the great distances in space making communication between civilizations impossible. Kind of the opposite to how on earth we have communication hotlines between nuclear powers to be able to de-escalate a situation.
[1] https://www.goodreads.com/book/show/23168817-the-dark-forest