For everyone who says "I can't prove X but you can't disprove it, so it's a matter of belief", I'd like you to read this article:
http://en.wikipedia.org/wiki/Russell%27s_teapot
It pretty much says that you can say the most craziest absurd thing ever and claim that people can't disprove it as long as you make enough excuses for it (ie: it's invisible! people haven't looked well enough for it! all traces of it were erased!). That's why usually the logical thing is trying to explain why X exists, not claiming it just does and challenging everyone else to prove you wrong.