Can astronomers tell whether stars and galaxies are made of anti-matter?
Not directly. In the answer to previous questions related to anti-matter in the universe, I mentioned that the only way we can tell if some object or gas cloud is made of anti-matter is to look for two tell-tail atomic ‘lines’ at energies of 511 keV and 931.4 MeV. These are produced when, in the first case, electrons and anti-electrons collide and annihilate. The second line, which is over 1800 times the energy of the ‘positron’ line, is the gamma-ray line produced when protons and anti-protons annihilate. To see whether any large- scale contact between matter and anti-matter is occurring in the Milky Way or beyond, you just have to search for these two lines in the X-ray and gamma-ray regions of the electromagnetic spectrum. Astronomers have done so over the last 3 decades with instruments of increasing sensitivity and have failed to find any evidence for them except in a few curious objects; one of which is called the ‘Great Annihilator’ located towards the center of our own Milky Wa
