Scientists at CERN have announced that they've been able to trap 309 atoms of antihydrogen for over 15 minutes. This is long enough that soon, they'll be able to figure out whether antimatter obeys the law of gravity, or whether it's repelled by normal matter and falls "up" instead. It would be antigravity, for real.
While it's never been tested experimentally due to how difficult it is to create and store the stuff, it's disappointingly likely that antimatter will fall "down" just like regular matter. The thinking behind this is that antimatter (despite the "anti-") is made of regular ordinary energy, and even if it's got an opposite charge, it should still obey the same general rules as matter does. Antimatter falling up would mean a violation of the law of conservation of energy, among other things.