Why? would you say, quantum mechanics theory is a universal truth, a fact, or just a very good guess? Because general relativity is also a theory, but both theories cannot be true in our universe as they contradict.
It's the same with almost any natural science theory. Theories try to be very good aproximations of the truth, until a better guess comes out.
Everyone, please stop pretending you aren't intentionally assigning different meanings to the same words. Put substance in your disagreement, not wordplay.
What you mean by "guess" isn't how any scientist would interpret "guess" in this context, and that what you call a "guess" is a rather glib and incomplete representation of all the things that comprise a "theory."
It's the same with almost any natural science theory. Theories try to be very good aproximations of the truth, until a better guess comes out.