That's what particle physicists are asking after they reported their first direct measurements, published in the current issue of Nature Communications, of gravity's effects on the antimatter equivalent of hydrogen. 

The measurements were taken at CERN, the huge particle physics laboratory on the French-Swiss border. CERN's  Antihydrogen Laser Physics Apparatus, or ALPHA, captures positrons and antiprotons, which are just like regular electrons and protons except that they have opposite charges and quantum "spins," in a vacuum chamber where they combine to form antihydrogen. 

When antimatter comes into contact with ordinary matter, the two obliterate each other in a flash of radiation and other subatomic smithereens (which is why we should all be grateful that Isaac Newton was never struck on the head by an antiapple). So, to get a good look at the antihydrogen before it vanishes, the ALPHA scientists trap it in a magnetic "bottle" that takes advantage of the antiatoms' magnetic properties to hold them in place.