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.
As this came up again it is worth clarifying that there is a misconception in the title:
Could the outer structure of the Universe be made from Antimatter?
There is no outer structure of the universe in the way the questioner envisages: the edge of the universe is where each of us is.
General relativity is a four dimensional, three space one time, theory, and the beginning of the universe happened at what is every point in the present day universe, by construction of the Big Bang model, which successfully describes all data.
So any antimatter at the edge of the universe would be detectable where we are now, by the annihilation channels and radiation. Observations and measurements show that there is little antimatter around us, even as scales grow, from planetary to galaxies.
There are 3 main categories of theories regarding antimatter's interaction with gravity:
- Antimatter is gravitationally attracted to both itself and normal matter.
- Antimatter is gravitationally attracted to itself, but is repelled by normal matter.
- Antimatter is gravitationally repelled by both itself and normal matter.
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.
Here's another possibility: Anti-matter decays at a slightly faster rate than matter. In the beginning, particles and energy ejected by decaying anti-matter would have had a head start into "space" right ahead of matter/anti-matter annihilation, leaving behind an equal amount of matter that then formed what we think of as the universe. At that time, particles of matter would have been relatively close to one another, a force overcome with the expansion of space, which may in part be due to the pull of anti-matter on matter. Over time, with the expansion of the universe and greater distance between particles of matter (we would have to assume an ever expanding central area where there is no matter or antimatter), that outer antimatter ring may have an ever greater pull on matter. I think of a bunch of magnets in a central area surrounded by an outer ring of magnets. Those inner magnets would be very attracted to one another in a jumble of N/S poles. If some force caused the whole contraption to expand and separate, eventually the inner magnets and outer magnets may become more aligned in their opposing poles and may draw faster and faster toward one another.