If something exists it should be easy to prove. There should be some form of sign of it.
This is absolutely not true. Things can exist without being accessible to you directly in a manner that makes it easy to prove their existence.
On the other hand it is hard to disprove the existence of anything at all. How do we know there is not some teapot in outer space?
Proving non-existence is not always hard. If we were arguing about the food in your fridge and I were claiming you had food in your fridge when you did not you could easily prove me wrong by just showing me the contents of your fridge.
More importantly, why does the hardness of doing a thing give you special status to make claims without proof? Seems like you are artificially constructing rules here solely because they benefit your position.
We can’t. But that is no reason to believe there is one.
The universe is massive. There are teapots here. Why is it not plausible to believe some other alien race would not also construct some kind of teapot? Also, consider the fact that all teapots here on earth are literally teapots in “outerspace” in some sense.