Operational Hiddenness and Gravitational Unhideability: Why the Universe Admits No Perfectly Dark Objects

We propose a fully operational definition of information: the information content of a physical system is the complete set of outcomes obtainable by all measurements that an external observer can perform on it. Information cannot be destroyed, but it can be \emph{hidden} --- made inaccessible to a given observer. Motivated by the global conservation of information, we introduce a quantitative measure of hiddenness \(H(S)\), the fraction of a system's \emph{classical (geometric)} information that is inaccessible to a distinguished observer. We establish a universal physical bound: in any universe with long‑range gravity, \(H(S) < 1\) strictly for all systems. Perfect hiding would require perfect decoupling, which gravity forbids. Using quantum estimation theory, we derive a minimal leakage of information through the gravitational field and show that even the darkest known objects --- black holes --- satisfy \(H < 1\) due to the unavoidable imprint of their macroscopic parameters (mass, charge, angular momentum) on the spacetime geometry. The bound provides a new operational perspective: while the qualitative no‑hair theorem already asserts that certain classical parameters are visible, our bound quantifies the fraction that must be accessible and shows that perfect hiding of classical information is impossible, even for objects where microstates remain overwhelmingly dark. A complete theory of quantum gravity may reveal further unhideability mechanisms, strengthening the bound.