I am always intrigued by the notion that 'aliens' must of course have a physical form, because we do.
What if they are simply beams of light with no physical form, and therefore nothing to prevent them from travelling from one end of the universe to another in a nanosecond.
It is perfectly possible that aliens have, and do visit us, but leave no sign of their presence, and never have, and never will.
I find that far easier to believe than the popular notion that aliens must visit other planets via a 'ship' because at the moment, that is what we would have to do.
It's always interesting to observe how the prevailing thoughts at any one time come to be disproved - we were supposed to be teleporting by now, and waited on by robots, if we fulfilled the notions that people in the sixties had for us.
Similarly - films like Jurassic Park perpetuate the notion that all dinosaurs were shades of grey and roared a lot.
What if they were covered in rainbow shades of felt and squeaked like mice?
We remain in ignorance, - maybe our visitors prefer us that way.