You seem to be applying the layman's definition of a theory. "In common, everyday language when you have a theory it means you simply have a guess. It doesn't have to be supported by evidence or fact." In everyday conversation, one would equate a "theory" to a "guess". For example, 'I have a theory on why so-and-so is dating who' and 'I have a guess on why so-and-so is dating who' can be used interchangeably.
A theory, in science, has been tested and looked over many, many times by numerous individuals and groups in order to verify it's authenticity. A theory, in science, is held by the majority of scientists to be the strongest idea backed by facts and reasoning. Simply dismissing the BBT as a "theory" is hogwash, because the word "theory" can mean very different things in different contexts. If the BBT was called the Big Bang Hypothesis (scientific term for "a guess"), you'd be right in your dismissal of it. However, as it is, the BBT is not simply a "theory", it has a strong case for it throughout the multitude of studies that have been done in order to seek out our origins.