Observations have found black holes spanning ten orders of magnitude in mass across most of cosmic history. The leading black hole model, the Kerr solution, is however provisional because its behavior at infinity is incompatible with an expanding universe. Recent work on black hole models with realistic behavior at infinity predicts that the gravitating mass of a black hole can increase with the expansion of the universe independently of accretion or mergers, in a manner that depends on the black hole’s interior solution. Here we test this prediction by considering the growth of supermassive black holes in passively evolving elliptical galaxies over 0 < z < 2.5. We find evidence for cosmologically coupled mass growth among these black holes, with zero cosmological coupling excluded at 99.98% confidence. The redshift dependence of the mass growth implies that, at z < 7, black holes contribute an effectively constant cosmological energy density to Friedmann’s equations. The continuity equation then requires that black holes contribute cosmologically as vacuum energy. We further show that, within current observational limits, black hole production from the cosmic star formation history gives the cosmological constant value measured by Planck. We thus propose that stellar remnant black holes are the astrophysical origin of dark energy, explaining the late-time onset of accelerating expansion.