Intelligent aliens do not exist in this universe. Full stop.
Parallel universes may or may not exist. If they do, they may or may not include aliens. The method of split between universes, also unknown, may generate finite or infinite numbers. I can imagine these universes including aliens only, humans only, humans and aliens, being totally lifeless and other possibilities including having gone extinct.
My guess; and it is a guess, is that we may one day be able to prove their improbability or likelihood but never the actual content within them. This could be wrong but is my best guess. It is unrelated to the content of this universe which includes no aliens.
I do find it interesting to ponder what would create a "split" and what would not. Before life existed was there only one universe full of inanimate objects bouncing around? Were these splits only generated by the action of beings exhibiting some sort of free will? If so, is that because it is all a simulation? If not is it because of something else that causes inanimate objects to act differently despite identical conditions? These are all interesting to me and worthy of investigation.
The concept of aliens existing in this universe is boring and worth not a cent of further inquiry. That doesn't mean stop studying the universe at all. It means stop looking for something you'll never find.