It's hard to prove the existence of anyone.... like it's difficult to prove an existence of any person you haven't met yet. You can have lot of evidence but you will always end in your initial presumption and many people will support you in that based on their experience. And you can be either satisfied with that or not. If not you can still meet and get know any person but you need to seek that person, otherwise you have no chance. If you don't seek you will end knowing nothing with just a pure speculation in harmony with our internal preference. Dr. Kaku is seeking a lot, he has a good chance to find.