No. 47: Sep-Oct 1986
The clumpiness of the universe described above assumed ordinary matter. Perhaps there are inhomogeneities on a different, more basic level -- matter vs. antimatter.
According to one popular theory, the universe began with equal amounts of matter and antimatter. If so, where did all the antimatter go? We assume we observe a universe that is virtually 100% matter. Of course, we cannot really tell for certain because an antimatter galaxy would appear to us just like a galaxy composed of ordinary matter. The only clues revealing substantial pockets of antimatter would be the annihilation radiation produced where matter and antimatter regions rubbed against one another. The two types of matter always annihilate one another in bursts of very distinctive radiation.
Well, there seems to be at least one region of antimatter near the center of our galaxy. The HEOS3 satellite and ballon-borne instruments have pinpointed a source of 511 kev gamma rays that can come only from a spot where electrons and positrons are mutually annihilating each other. (The positrons are antimat-ter analogs of electrons.) This region of mutual destruction is about 1013 kilometers across. Is it a pocket of antimatter left over after the Big Bang that a sea of surrounding matter is finally wiping out, or is it newly created antimatter in the vicinity of a black hole? No one knows. The mystery has deepened with the discovery that the intensity of the annihilation radiation varies with time. Something strange is going on out there.
(Anonymous; "Galactic Positronium Mystery Deepens," Science News, 130:40, 1986.)