I don’t believe there are aliens - as in, beings from other planets.
Now, you raise an interesting point - could “aliens” be fallen angels? That’s certainly possible, or as you suggest, demons, and other principalities of the Fallen One.
But, there’s nothing I’ve read in the Bible that would indicate beings from other actual planets. YMMV, and of course, I could be wrong. Won’t know til I’m dead and in Heaven.
We know aliens exist, as there is too much proof of flying craft that we could never design for the next 500 years, at least. It’s only a question as to what it really could be.
The Bible only gives us what we need to know. We don’t need to know anything else about life outside this planet, for now. I do believe that, in eternity, the universe will be ours to explore and everything in it for us to get to know, what ever that will end up being.