The phrase "a theory which" is correct and usable in written English.
It can be used when introducing or describing a specific theory in a more formal or academic context.
Example: "The study presents a theory which explains the relationship between climate change and biodiversity loss."
Alternatives: "a theory that" or "a theory stating that".
Your view was a theory, which was however not presented as a theory but as a near certainty.
Katherine Frank has a theory, which has sent her foraging across oceans in quest of wild geese and red herrings.
Professor Andrea Galeotti will use the funding from the European Research Council to develop a theory which could help both companies and governments.
As for pi, he had a theory, which he'd never shared with anyone, that its ubiquity had something to do with dark matter.
Ms. Kirschner, 52, has developed a theory, which she said applied not only to herself but to most anyone buying gadgets.
"I have a theory which I suspect is rather immoral," George Smiley said in John le Carré's 1974 classic, "Tinker, Tailor, Soldier, Spy".
The BBC reported that time-frames indicate the poison was ingested in Belgium, a theory which has been supported by an independent toxicologist.
They in return should make no objection to the teaching of evolution in science classes as a theory, which indeed it is.
If confirmed by other groups, the discovery would count as the strongest evidence yet for cosmic inflation, a theory which says that the universe went through a period of extremely rapid expansion soon after the big bang.
I've even seen a theory which states that breasts evolved to remind men of their mother's buttocks, a familiar and reassuring site for tree-climbing primates, and a notion to make every male reading this recoil in oedipal disgust.
Lewis is most celebrated for his "modal realism", a theory which argues that possible worlds are not just a concept for explaining possibility and necessity, but as real as our own universe.
