Physicists are increasingly certain a mysterious force is driving the universe apart. If only they knew what it was.
For astronomers, 2003 brought some answers, more questions and a deepening conviction: Something strange is happening to the universe. In February a satellite operating a million miles from Earth made a series of measurements that were as baffling as they were precise. A mysterious repulsive force called dark energy accounts for 73 percent of the entire mass-energy of the universe, the Wilkinson Microwave Anisotropy Probe (WMAP) found; 23 percent consists of invisible dark matter, and only 4 percent of the universe is ordinary matter and energy.