a belief that a god does not exist is an act of faith every bit as much as a belief that a god does exist is. neither position can be proven to be true or false.
*shrug*
*yawn*
*burp*
The belief that a Yewy-yewy does exists versus the belief that a Yewy-yewy does not exist is mainly based on the definition of what a "Yewy-yewy" is.
Whatever is not defined can not be proven to exist. Nor is there a requirement to disprove it.