We propose that Maximally Entangled Nonspace (MEN) boundaries are a universal fea- ture of quantum-gravitational systems, forming when entanglement capacity saturates. These boundaries serve as initial data surfaces in cosmology and as partially reflective layers within black holes. Grounded in established principles—Quantum Extremal Surfaces (QES), the Quantum Focusing Conjecture (QFC), and Boundary Conformal Field Theory (BCFT)—MEN yields a unique Lorentzian reflectivity law controlled by a single parameter: the entanglement gap ∆. This law produces falsifiable predictions across domains: clustered gravitational-wave echoes within narrow time-frequency corridors, and cosmological spectra with tightly bounded ranges for scalar tilt \( n_s \) and tensor-to-scalar ratio \( r \). By requiring cross-domain consistency of ∆, MEN offers a stringent unification test: the value inferred from black hole echoes must match that required by cosmological spectra. Failure in either domain falsifies the framework. Success would provide a conceptually economical and empirically grounded principle linking black hole interiors to the origin of cosmic structure.