If the thing you are claiming exists does not interact with anything else in any way, how are you proving that thing exists in the first place? It's basically a circular argument. You make a claim that something exists yet then state that it is impossible to prove that the thing exists. It's just one big circle of "trust me, bro". The question isn't "does it exist?" the question is properly, "how do you know it exists in the first place?" You need to prove that the unicorns exist before expecting people to accept that they exist. Their existence is not a given.