We show that when standard quantum vacuum fluctuations are subjected to boundary conditions imposed by a finite causal universe, specifically the region accessible since the time of recombination, bounded by the distance to the cosmic microwave back ground (CMB) at ∼45.7 billion light-years (∼8.65 × 1026 m) and assuming a discretized space structure at the Planck length scale (∼1.616 × 10−35 m), the resulting suppression of vacuum modes leads to an effective energy deficit. This deficit closely matches the observed dark energy density in the present (∼5.3 × 10−10 J/m3) and the evolution in the past for different redshifts, without introducing any free parameters or exotic new physics. The calculation builds on established physical principles such as the Casimir effect and relativistic causality. Although this article does not aim to explain dark energy, the observed numericalcoincidence suggests a possible connection between suppressed quantum vacuum fluctuations and dark-energy density.