The phrase "a theory does not exist" is correct and usable in written English.
You can use it when discussing the absence of a particular theory in a specific context or field of study.
Example: "In the realm of quantum mechanics, some argue that a theory does not exist to fully explain the behavior of particles at the subatomic level."
Alternatives: "a theory is absent" or "no theory exists".
Such a theory did not exist at the time.
To try to imagine my child being terminally ill or suffering "unbearable physical or mental suffering that cannot be alleviated" puts me in an emotional place where theory does not exist.
For integro-differential and other classes of nonlocal operators, inverse problems are more difficult to investigate, and the main classical methods (transformation operator method and the method of spectral mappings) either are not applicable to them or require essential modifications; and for such operators, the general inverse problem theory does not exist.
Our weak transversality hypotheses allow us to justify expansions where even a formal theory did not exist before.
The fact that social contract and hazard management algorithms provide better explanations for performance in their respective domains does not mean that the content-general logical procedures posited by relevance theory do not exist, or that relevance effects never occur.
Despite the existence of numerous anecdotal and observational reports on this mechanical theory of claw toe pathogenesis and despite wide acceptance of this mechanism in the diabetic foot literature, experimental data to support this theory do not exist.
And so if you suddenly traveled to cities that, according to this theory, didn't exist or hadn't yet had time to put themselves together, the result was the phenomenon known as jet lag, which arose not from your exhaustion but from the exhaustion of the people who would still have been asleep if you hadn't traveled.
But these theories don't exist in a vacuum; there are people who profit from promoting these things.
Such a girlfriend does not exist.
Such a logjam does not exist.
