There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.

This quote is saying that some people believe that if anyone ever figures out the true purpose of the universe, it will disappear and be replaced by something completely different and unexplainable. Others suggest that this might have already happened. In other words, someone may have already figured out the purpose of the universe, but that knowledge has since been lost.