The Theory of Gravity? The Globular Earth Theory? The Theory of Evolution? The Theory of Intelligent Design? The General Theory of Relativity? The Special Theory of Relativity? The Theory of the Evolution of Species?

If E=MC^2 is only a theory, then I guess it's problematic that them fission bombs don't work, huh?

Nah. A Theory (capital T) has almost the force of a Law. And I didn't know that gravity was theoretical....