What?
The infinity of space?
The probability of other dimensions?
Energy manifesting as matter?

I would go first with the dimensional theory, including gravity, magnetics, hyper-space, time, and our baby steps into understanding any of them, especially in relation to each other. I would attribute these "sightings" to any of the above rather than any other planets and long voyages or "aliens"; but I could be wrong.

Whatever's around has been around for a long time, and it manifests as non-physical energy in the physical. Show me a crashed saucer. Prove it to me.
I still say the jury is out until someone puts a saucer on display.