  We consider that the universe is trapped in an excited vacuum state and the
resulting excitation energy provides the observed dark energy. We explore the
conditions under which this situation can arise from physics already known.
Considering the example of how macroscopic QED fields alter the vacuum
structure, we find that the energy scale 1 meV --- 1 eV is particularly
interesting. We discuss how dark energy of this form is accessible to
laboratory experiments.
