[elil17]

To my understanding, the researchers released antimatter particles with detector plates above and below them. The particles started out traveling in random directions. Some of the particles hit the top, some hit the bottom. They saw that more particles hit the bottom than the top.

If the particles had "anti-gravity", they'd be repulsed by the large mass of the earth (instead of attracted), and you'd have expected more to hit the top plate than the bottom plate.

The researchers also added a magnet to the top designed to cancel out the downward force from gravity, and they hit the top and bottom plate at even rates.

[pvaldes]

Hum, How can they be sure that what is hitting the plate is still antimatter? (Or only antimatter?)

[ben_w]

Positrons react with electrons to produce a distinctive pair of 511 keV photons travelling in opposite directions in the frame of reference of their collision.

There's also a much more complex mess that happens when protons react with antiprotons.