MARINA DEL REY, Calif., Feb. 20 — A dark, unseen energy permeating space is pushing the universe apart just as Einstein predicted it could in 1917, according to striking new measurements of distant exploding stars by the orbiting Hubble Space Telescope. The energy, whose source remains unknown, was named the cosmological constant by Einstein. In a prediction he later called "my greatest blunder," but which received its most stringent test ever with the new measurements, Einstein posited a kind of antigravity force pushing galaxies apart with a strength that did not change over billions of years of cosmic history. Theorists...