The universe is not only expanding — its expansion is accelerating. After decades of searching, physicists still do not know what is causing it. The name they gave the mystery, dark energy, is less an explanation than a placeholder for one of the biggest gaps in modern cosmology.

That the universe is expanding has been known for nearly a century. Galaxies are flying apart, and the more distant ones recede faster, as Edwin Hubble worked out in the 1920s. The shock came in 1998, when two teams measuring exploding stars found that the expansion is not slowing under gravity, as almost everyone expected, [...] The post The universe is not only expanding — its expansion is accelerating. After decades of searching, physicists still do not know what is causing it. The name they