This is generally not how scientists, if they are being careful, use the word "theory".
What you describe, the idea that two things are connected, is not a theory. That's a hypothesis. A hypothesis is a claim about the world, and it might not (yet) be equipped with an acceptable explanation for why it should be believed.
A theory would be a formula or equation or perhaps a process which is consistent with a set of information and allows scientists or mathematicians to calculate more information. It's a system whose consequences you can work out on paper, or in a computer, or in your head. But notably, a theory need not have any bearing on reality. You can develop a robust theory in all its mathematical glory and never find or expect to find anything like it out in the universe. It is a theory nonetheless, because you can work with it and explore it for it's sake.
Now, certainly, we have developed some theories of dark matter over the years and hypothesized that they are candidates for explaining the real world. There are many such theories. And, for each one, some scientists hypothesize that that one might be an accurate description of the world.
a hypothesis is the model (alone) for how things might be
a theory is the hypothesis + corroborating observation
if you have a set of observations and put forth a hypothesis from it (versus a hypothesis that has no observations backing it), it is automatically a theory. it may not be a good one but it is one nonetheless.
What you describe, the idea that two things are connected, is not a theory. That's a hypothesis. A hypothesis is a claim about the world, and it might not (yet) be equipped with an acceptable explanation for why it should be believed.
A theory would be a formula or equation or perhaps a process which is consistent with a set of information and allows scientists or mathematicians to calculate more information. It's a system whose consequences you can work out on paper, or in a computer, or in your head. But notably, a theory need not have any bearing on reality. You can develop a robust theory in all its mathematical glory and never find or expect to find anything like it out in the universe. It is a theory nonetheless, because you can work with it and explore it for it's sake.
Now, certainly, we have developed some theories of dark matter over the years and hypothesized that they are candidates for explaining the real world. There are many such theories. And, for each one, some scientists hypothesize that that one might be an accurate description of the world.
But, no, the idea of dark matter is not a theory.