|MadSci Network: Astronomy|
You're right that electromagnetic forces make gravity an extremely difficult force to measure on laboratory antimatter. Experimentalists are getting much closer, and will probably be able to perform the experiment in just a few years, but it hasn't been done yet -- so there is a slight possibility that antimatter might fall up. (If you have access to a physics library, look for John Eades' article in the Jan 1, 1999 issue of 'Review of Modern Physics')
But the consensus is that antimatter will fall down, and there are some very good reasons for believing this to be true. The best reason comes from general relativity, which predicts that clocks should run at different rates in different gravitational fields. If gravitational fields for protons were opposite those for antiprotons, then a neighboring proton and antiproton would have slightly different 'clocks'. But an experiment in 1991 (Physical Review Letters, v.66, p.854) proved that this was not the case, strongly suggesting that gravity is not reversed for antimatter. Technically, though, this was a measure of inertia, not gravitation. General relavity assumes the two should be the same, but I suppose the argument isn't ironclad -- yet.
The other reason no one thinks that antimatter galaxies are out there is that the matter/antimatter imbalance is already explained by an asymmetry in the laws of physics -- an asymmetry that has been carefully studied in particle accelerators. In the early universe, this asymmetry led to slightly more matter being produced than antimatter, and subsequent annihilations led to the matter-dominated universe we live in. This asymmetry may seem strange, but it has been carefully documented in the lab.
Your instincts are good -- it makes intuitive sense that an equivalent amount of antimatter should be somewhere -- but consider this. The laws of physics are not symmetric under charge (C) exchange, but rather under something called CPT symmetry. This stands for Charge, Parity (mirror-image), and Time; flip all three and the laws of physics stay the same. Taken to an extreme which most physicsts don't like to think about, the whole universe should be CPT symmetric. That would mean that an antimatter-dominated phase of a closed universe might exist near the end of the universe, right before the Big Crunch. In other words, the Big Crunch at the end might be a CPT-reversed version of the Big Bang! In this case, the 'missing' antimatter would exist elsewhere in time (in the far future), not elsewhere in space.
The idea of a closed, CPT-symmetric universe originated in the 60's with a physicist called Gold, and was later discussed in Hawking's "A Brief History of Time". Hawking gave up on the idea shortly after he wrote the book (he now calls it his greatest blunder), and few people today take a CPT-symmetric universe seriously. Still, I think the concept deserves more thought, especially because there is a little-discussed type of CPT-symmetric universe which seems to solve most of the usual counter-arguments.
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