As a proposed explanation:
* The theory of relativity revolutionized our understanding of gravity.
* She developed a theory about the causes of the economic downturn.
* His theory was based on years of research and observation.
As a set of ideas:
* The film explores the theory of the interconnectedness of all things.
* He presented his theory of music in a lecture at the university.
* The professor taught us the theory behind the scientific experiment.
As a concept or idea:
* In theory, it sounds like a good idea, but in practice, it's difficult to implement.
* Her theory was proven wrong by the latest scientific findings.
* He had a theory about why the car wouldn't start.
In a more casual way:
* I have a theory about why she's always late.
* Let's just call it a theory for now.
* My theory is that everyone likes pizza.