Could aliens exist in another dimension?

Maybe in theory — but it depends on what you mean by “another dimension.”

• Extra spatial dimensions: Physics sometimes allows more than the 3 space dimensions we notice, but these are usually thought to be tiny or curled up. There’s no evidence that ordinary life could exist there.
• Parallel universes: If “another dimension” means a separate universe, then life is possible in principle. But this is speculative, and we have no proof other universes exist.
• Hidden or dark sectors: Some theories imagine matter that interacts very weakly with ours. Alien life there is a remote possibility, but there’s no evidence for it.

Bottom line: Aliens in “another dimension” are possible as a theory, but not something science has confirmed.