Eryn Brown, reporting for the L.A. Times on CERN’s ALPHA experiment:
Indirect evidence suggests that gravity’s effects are the same on matter and on antimatter. But gravity isn’t especially well understood, so there could be “a window of opportunity for other strange things to happen,” Fajans said.
The only way to find out for sure is to test gravity’s effect on antimatter directly. That’s where ALPHA comes in. The experiment uses strong magnets to trap antihydrogen atoms, which are formed by combining an antiproton and a positron.
Albert Einstein allowed for anti-gravity in his equations, though he had no way of knowing whether such a repulsive force existed. You can learn more about Einstein’s and others’ theories of anti-gravity in “The Fabric of the Cosmos: What is Space?”