Cern recently stated that antimatter may be repelled by matter, much like the opposite effect of Gravity. So is it possible that antimatter is actually repelled to the edges of the Universe to create a sort of outer-shell, something that allows the expansion of the Universe into nothingness..
If there were a matter-antimatter boundary, we would observe gamma radiation from the annihilation events on the boundaries. We do not observe such radiation events.
There are 3 main categories of theories regarding antimatter's interaction with gravity:
Your question seems to be assuming #3 is correct. However, concepts #1 and #2 are more popular.
Theory #3 implies that, with antimatter, one force (gravity) is inverted from the way it normally works, while the other forces (EM, Strong, & Weak) are not inverted. Why would only one force work differently? This inconsistency is hard to accept. To clarify:
A particle of antimatter may have the opposite EM charge as a corresponding particle of matter... but the EM force itself does not work oppositely. It works the same, whether we're talking about matter or antimatter: What I mean is that like charges always repel and opposite charges always attract. This is true whether you're looking at a hydrogen atom (proton + electron) or an antihydrogen atom (antiproton + positron).
So in antimatter, if the EM force isn't inverted, why would any other force (like gravitational interaction) be inverted?
And furthermore, we also know that the Strong Nuclear force works the same way with matter and antimatter. Otherwise the 3 antiquarks that make up the antiproton would not stay bound together.
That said, it's simply unknown at this time whether antimatter would fall up or down. Since the quantities we can produce are so small, it's not simple to determine how gravity affects these particles.
Dr. Jeffrey Hangst from Aarhus University in Denmark was the Scientist collaborating with CERN to trap antihydrogen particles. You may have read his recent paper where they were able to hold antihydrogen for 1000 seconds.
Dr. Hangst has designed another experiment which will determine experimentally whether antimatter would fall up or down in the gravitational field created by matter (earth).
Unfortunately CERN is in the midst of a maintenance and upgrade period, and won't be able to conduct the experiment until 2015. Here is a radio interview with Dr. Hangst about this exact topic.