Quite, so you agree that the possibility of things that we currently have no empirical evidence of and currently have no practical scientific way of testing may still exist, because our current limited knowledge of the Universe doesn't effectively negate those hypotheses?
I love this argument. It occurred to me a long time ago that it's effectively not possible to reason the concept of atheism that some people hold. The empirical problem is that if a rationalist can think something into existence, you can cite there is no evidence for it, but from a rationalist point of view once such a concept materialises, you completely lose your ability to reason it out of existence (the Descartes problem). This is why you can only ascribe to disbelief in a concept. No one can KNOW there is no God, they can only posit they don't believe there is one. It's exactly the same with a UFO, the second someone conceptually imagined the possibility of one, there is now no way we can ever disprove the notion, whilst we are yet to prove it.
I think we could well have already seen them, but the evidence would have to be utterly unequivocal for it to be held as a conviction that they exist. I feel there's nothing wrong with believing in UFOs.
I for one, for mainly sentimental reasons, hold a belief that there must be extra-terrestial life. I don't know what form it make take or if it's capable of space travel, but my sci-fi fuelled imagination would like to believe it's possible so I think I just hold the conviction in an imaginative capacity.