Empty space only looks empty. Zoomed out, galaxies are separated by vast voids. Zoomed in, those same voids behave like a restless medium, filled with fields and particles that never quite sit still.
Quantum field theory says every point in space hosts fields that retain zero-point energy even in their lowest state. These fields generate quantum fluctuations that constantly create and annihilate virtual particle pairs. Most of these events cancel statistically, yet they leave measurable fingerprints, from the Casimir effect to slight shifts in atomic energy levels.
On larger scales, the cosmic microwave background bathes every cubic meter in relic photons, a faint radiation field that still carries information about early cosmic entropy and density variations. Layered on top is dark energy, often modeled as a vacuum energy density that drives the accelerated expansion of space and dominates the global energy budget, despite being extremely dilute.
Gravitational fields, neutrino backgrounds and stochastic gravitational waves add further contributions. The result is a universe in which the apparent emptiness between stars and galaxies functions less like a blank stage and more like an active infrastructure, quietly shaping how structure forms, light propagates and time evolves.
