From the space.com article:
Antimatter, which annihilates matter upon contact, seems to be rare in the universe. Still, for decades, scientists had clues that a vast cloud of antimatter lurked in space, but they did not know where it came from.
The mysterious source of this antimatter has now been discovered — stars getting ripped apart by neutron stars and black holes.
Read the article here.
More info about antimatter here.