I can't say I am sure, so I'll just give stuff that I've heard.
One theory that may relate to what you say here is that Aliens are actually the fallen angels (demons). I've heard it from some christian conspiracists and read it somewhere. However the theory is, that when the angels fell from heaven and entered our physical universe, they ended up getting bound to the laws of the universe. Which means these spiritual-non matter entities ended up becoming matter.
So since angels are not made in the likeness of God, their physical form isn't exactly human looking, but as the aliens that have been popularly illustrated. Additionally, since they fell from grace and became disconnected to God's grace, they also lost their powers (because the Holy Spirit was the source of it) hence relying on advanced technology (UFO's). When they entered our universe, they ended up creating their own civilizations on other planets (because why fall automatically to earth when there are other planets in our solar system).
I forgot the rest of the details, but a lot of it ends up leading to the conspiracies involving the existence of the Illuminati, in where some of these Aliens/Angels have made human costumes to pose as celebrities and politicians through out time, which explains why there are photographs or portraits people dating from the 1400's-1800's who look so much like
modern day celebs.
Again, i'm not stating this as truth that people should consider.. it's just an interesting/entertaining conspiracy theory and i understand that it does sound crazy.