Conservation of energy/mass is a rule for the known universe. If we knew all the rules for the dark part of the universe, we wouldn't even be having this discussion.

Anyway, the way the math works out, it is impossible for a known-universe person to even measure the effects the known-universe has on the dark-universe. The numbers can't come back to the known-universe without an additional input from it, and they would be explainable with another hypothesis that did not include a dark-universe. It way be that all effects we observe to be random are actually perfectly predictable to a dark-universe observer.