While in physics, dark matter cannot be observed directly, it is studied through its effects on the broader system with which it exists.
Similarly, we could use resource flows around unobserved or unobservable capital.
Private firms do business with on-private firms, and while this won’t provide the full picture about them, it will provide hints on how to refine data about private firms.
Organized crime cartels and syndicates are already providing a ton of data…. But to the state entities that are meant to be investigating them. This data would include resource inputs, crime statistics, social networks, and probably a plethora of sources I can’t think of right now.