|MadSci Network: Astronomy|
A difficult question to answer, and one which I might have to sidestep! As you may know, once antimatter collides with normal matter it annihilates, releasing energy and destroying itself in the process (along with the normal matter!). As our galaxy has not annihilated, we can therefore deduce that our local reigon of space is made of "normal" matter.
This logic is extended to the rest of the universe, and astronomers do not expect to find antimatter galaxies or stars. That said, if they did exist it would be almost impossible to tell from a distance!
The greatest "source" of antimatter was the big bang! Theories of particle physics suggest that almost equivilent numbers of matter and antimatter nuclei were created and annihilated in the first fractions of a second. However, there must have been an imbalance (of about one particle in a thousand million) to leave us with matter today, and finding out why that is has been a major problem in cosmology for years.
[Moderator's Note: Even in our local area of the universe, some antimatter particles - particularly positrons - are created. It's just uncommon! Fusion reactions in the sun actually create some trace amounts of antimatter, which of course immediately annihilate. Quasars with radio jets may create positrons (although nobody's sure about that yet). Cosmic ray protons and electrons can also create positrons when they collide with photons or protons in the interstellar medium.]
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