A REPULSIVE FORCE IN THE UNIVERSE seems to be at work on a cosmic scale, at least partly neutralizing the attractive force of gravity. Astronomers have arrived at this conclusion after a series of observations of distant supernovas showed that the expansion of the universe has not only not slowed (through the mutual attraction of galaxies) but seems to be accelerating. The most recent data to support this view were reported by an LBL group in January at the AAS meeting in Washington (Update 355) and by the multi-national High-z Supernova Search Team two weeks ago in California at a symposium on dark matter (Science, 27 February 1998). Several theories attempt to explain the positive push; one popular model sees the effect as coming from a huge reservoir of energy hidden in the universal vacuum. An early precedent for this notion was Einstein's use of an antigravity fudge factor called the cosmological constant in his gravitational equations. (See also Science News, 28 Feb, and Updates 345 and 360.)