I would agree that aliens can be interdimensional, and right at this point in time, we know "atoms" to exist everywhere.
Not sure if you know atoms are now thought to appear and disappear they are not static (apart from the fact that they move), so the question is, where do they go?
It's also thought that we manifest "objects" like the chair, the computer your using and everything before we see it. If you left the room, the contents of the room would cease to exist, until you return. Tricky eh. :-)
The atom is thought to now move into and out of various dimensions space which could explain why UFOs are able to appear and disappear. If atoms are able to interact with our universe this way, why not whole bodies.
I have a theory on aliens/UFOs, our eyes only see in the visible light spectrum, it's quite possible to hide from us by cloaking in infra-red or high frequency. Morever an easy approach would just be to bend light which we can already do today.
This one isn't science fiction, its science fact, we know it as refraction.
Thanks for your contribution,