You can never prove God exists, you can never prove God does not exist. The essence of any complex system has similar properties - you can only prove what you can reproduce and you can only disprove that which can be disproven via reproduction. The higher the number of variables, the lower the predictability of the system. Its a pretty fundamental and obvious concept.

So if you don't like that simple glaring fact, don't get pissed off when people respond "I'm not sure". Absolute certainty in the terms you're talking about is a leap of faith, whether you think it is or not.