Antimatter at the edges of the universe
Theoretically, matter and antimatter were present in equal amounts soon after the universe was formed. Antimatter then reacted with matter and both annihilated. After the annihilation, more matter than antimatter remained. Before the annihilation, antimatter decayed slightly faster than matter (this phenomenon of antimatter has been verified).
When antimatter decayed, it would have ejected particles. A percentage of those particles would have been ejected away from the position where most matter resided and would not have reacted with matter. Because those particles had a head start, they would continue heading outward and nothing (no matter) would be able to catch up to them. Therefore, antimatter would be present at the outer edges of the universe. (Originally posted on PhysLink.com – Oct 10, 2002)
Over time, the gravity that causes galaxies to pull toward each other would weaken due to distance. The gravitational pull from antimatter would begin to attract matter more than matter would attract matter. If so, matter eventually will be pulled into antimatter and annihilate.