I think most academics studying dark matter are pretty upfront about not knowing what it is. The theories with the most fans tend to not be very specific about the source, but agree that it's most likely an "unknown something" with mass that causes gravity. There are some theories like MOND that instead postulate that there's no missing mass, but that our understanding of gravity needs to be altered at galactic scales.
In any case, it's a placeholder, not a specific theory. There are many candidate theories, but so far they're either untestable or the tests have had negative outcomes.
Just to put it in context, I'll share one of my favorite (even though I think it's one of the more outlandish and less accepted):
In the Multiverse/Multidimensional universe theory, all of these dimensions aren't necessarily in different universes. Some are of different dimensions, laid over top of ours, but we can't see them because we can't perceive the dimensions they exist in. The only dimension that bleeds into ours, that we can track so far, is gravity. This is what we call "Dark matter". It is actually gravity leaking over from an "Nth" dimension laid over our own that we can't perceive.
Again, as to the veracity I can't say. And I wish I remembered to author, but I don't =/ maybe I'll look it up later and make an annotation.
In any case, it's a placeholder, not a specific theory. There are many candidate theories, but so far they're either untestable or the tests have had negative outcomes.