Cosmic rays bring our planet a steady stream of protons, electrons, and other particles. As these collide with nuclei in the upper atmosphere, they create new particles, including antiprotons. And some of this antimatter is sticking around above our world.
The process that creates these antiprotons is pretty much exactly the same as how we create them in particle accelerators - high-energy collisions between fast-moving particles create daughter particles, which includes pairs of protons and antiprotons. Most of these matter-antimatter pairs meet and annihilate each other pretty much instantly, but what if a few antiparticles managed to escape interaction with ordinary matter?
Well then, we would then have a reservoir of antiprotons above our planet. Specifically, astronomers theorized that these antiparticles would get trapped in the Earth's magnetic field. Now, thanks to a five-year search by the probe PAMELA, we've at last found conclusive evidence that this is indeed happening.
The big break came from an area known as the South Atlantic Anomaly, which is a region of space where the Van Allen Radiation Belts are the closest to our surface. This anomaly is a known pain in the ass to anyone wanting to work in space. The International Space Station requires extra shielding just to protect its astronauts as it passes through it, and the Hubble Space Telescope has to be turned off every single time it passes through the anomaly...which is multiple times daily.
Using 850 days of data and focusing just on the 1.7% of the time PAMELA passed through the anomaly, astronomers discovered 28 antiprotons. That may not sound like much, but that's about a thousand times more than would be expected under normal circumstances, and it's enough for the PAMELA team to declare the South Atlantic Anomaly "the most abundant source of antiprotons near the Earth."
This belt of antimatter is small, and any threat it poses to those who pass through the anomaly is dwarfed by all the other things that make the region dangerous. Still, it's good to have confirmation of this theory, and it may well presage the discovery of similar antimatter reservoirs around our planet.