The universe is expanding, and observations show that its expansion has accelerated over cosmic time. Astronomers call the cause dark energy, a label for an observed effect whose underlying nature remains unknown.
The evidence is not a single strange measurement: distant supernovae, the cosmic microwave background, and the large-scale arrangement of galaxies all help map cosmic expansion. In April 2026, the DESI collaboration reported completing its planned five-year sky map; its first three years, combined with other datasets, had hinted that dark energy could change over time.
That hint is not a settled discovery. Those data alone fit the standard cosmological model, and the full-survey dark-energy results were still expected in 2027. Researchers are testing whether dark energy is a constant property of space, a changing field, or a sign that gravity or the model needs revision.