Antimatter is rare in the universe today. As far as we know, all relic antimatter produced in the big bang disappeared long ago in annihilation reactions with matter particles. What this means is that any antimatter particles that we can detect in the flux of energetic cosmic rays near Earth must have been created by “new” sources within our Milky Way Galaxy. (Antimatter particles from extragalactic sources are also conceivable, but they are exceedingly unlikely to make it to Earth before losing all their energy or annihilating.
The 400,000 positrons they have measured constitute the largest set of cosmic-ray positron data, increasing the total world sample a hundredfold. In addition, the range of the reported positron fraction extends out to a few hundred GeV, beyond the reach of previous experiments flown on high-altitude balloons  or space shuttles and satellites  (see 5 January 2012 Synopsis).