Theories are a way of predicting what we will witness.
Given what I said in #443:
Theories are laws governing phenomena, or models of reality that are composed of those laws. The models are always mathematical constructions. Being that the theory governs reality, anyone can test it. It must be falsifiable and not contradict other valid theories.
That is always possible, and failure to predict an observation correctly, renders the theory deficient, or outright wrong.
A theory is not however, limited to just predicting what will be witnessed. It is a physical law as above.