The universe is defying the simplest expectation: its expansion is accelerating, even though gravity draws matter together. The evidence came from exploding stars. Type Ia supernovae, whose brightness can be standardized, appeared dimmer than expected at their measured redshifts, indicating that they were farther away than a slowing universe would predict.
The name dark energy is less an answer than an admission of ignorance. In general relativity, a cosmological constant can drive accelerated expansion because its energy density remains constant as space expands, giving it an effective negative pressure. Gravity has not stopped working; matter still gathers into galaxies and clusters. Yet on the largest scales, dark energy appears to dominate. Its physical origin remains unknown, and whether it is truly constant is still under investigation.
That uncertainty has company. Dark matter seems to supply the extra gravity needed to explain galaxy rotation and the bending of light, but its identity is unknown. The possibility of extraterrestrial life remains open, with no confirmed detection. So does the question of why matter survived when the early universe should have produced matter and antimatter in nearly equal amounts. Cosmic inflation may explain the universe's large-scale uniformity, yet its cause and details remain uncertain. Black holes pose another puzzle: how can information be preserved if Hawking radiation carries a black hole's mass away? Each mystery tests a different edge of physics. Beyond the galaxies, the expansion continues; what drives it remains hidden in plain sight.
