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CERN Antimatter Experiment Produces First Beam of Antihydrogen
An anonymous reader writes "Matter and antimatter annihilate immediately when they meet, so aside from creating antihydrogen, one of the key challenges for physicists is to keep antiatoms away from ordinary matter. To do so, experiments take advantage of antihydrogen's magnetic properties (which are similar to hydrogen's) and use very strong non-uniform magnetic fields to trap antiatoms long enough to study them. However, the strong magnetic field gradients degrade the spectroscopic properties of the (anti)atoms. To allow for clean high-resolution spectroscopy, the ASACUSA collaboration developed an innovative set-up to transfer antihydrogen atoms to a region where they can be studied in flight, far from the strong magnetic field (scientific paper)."
'Weapons that don't work if there is matter between you and the target' are probably kind of a niche at present...
Note that it can settle in the lungs, if you inhale too much and can't exhale it you can suffocate. You won't feel it either because the CO2 will rise and exit the lungs. If you experiment with breathing it in, stand on your head after a few seconds and breathe out/in deeply.
Not a sentence!
Parent and GP's suggestions might work in SF, but in real physics, a powerful laser blast will just break the ordinary matter into smaller pieces, which will still be sitting in the way. As for the positrons, they won't quite annihilate the ordinary matter; some of them would annihilate with electrons, but most of them would convert neutrons into protons, if I remember the nuclear chemistry chapters of my bachelor's courses correctly. The resulting unstable cores would either decay or fission, but the products would still be ordinary matter, and no matter (pun not intended) how long you keep repeating this, there would still be a lot of ordinary matter left that cannot be converted to energy any further by bombarding with positrons.