[Frumious Bandersnatch]

I know this may be splitting hairs, but I believe that a theory can be used in making predictions, but the theory itself only describes a set of observed phenomona. For example the General Theory of Relativity describes the relationship between energy and mass. It does not predict anything. Using the theory, however, we are able to predict the ability to build an atom bomb.

[PatrickHenry]

General Theory of Relativity describes the relationship between energy and mass. It does not predict anything.

To the contrary, one of GR's predictions was famously tested when the predicted deflection of starlight was first observed during a solar eclipse.
Eclipse in 1919 helped confirm theory of relativity.