First, what is the motivation to incorporate a physics metaphor into CasP theory? For example, if “the quantum of capital exists as finance, and only as finance,” why are you looking to physics–and not accounting– for answers? Every corporate entity (whether private, public, for profit, not for profit, law-abiding or criminal) must nominally follow the same accounting rules for tax purposes. Whether or not you can see what’s in the books of private entities, you know what the accounting rules are. So, why can’t you build testable models based on the accounting rules?
Second, if it is appropriate to borrow metaphors from physics, is “dark matter” the right metaphor to use? From what I can tell, things like dark matter and dark energy are posited because, empirically, their absence implies a violation of the conservation laws physics calls thermodynamics. CasP theory, however, is not premised on conservation laws, and the fact that CasP theory to date has focused almost exclusively on the market capitalizations of publicly traded firms does not mean you need a theory of “dark capital” to understand capital as power in privately held firms. Yes, the fact that privately held firms don’t have market capitalizations may obviate the universality of certain assertions made by CasP theory, but it doesn’t obviate all of them, which suggests that the publicly traded firm, not the privately held firm, is the special case.
In any event, accounting is a system of conservation laws. Mirowski even argues that principles of accounting may have inspired the development of thermodynamics (see, More Heat Than Light.) This is one more reason to look to accounting for answers, not physics.