The difference between matter and antimatter is just the opposite spin directions (= direction of self rotation of electrons). After generation otf the universe there have been 50% electrons and 50% positrons. By further interactions by overlapping each overlapping will cause that particles with different spin orientations are partly turned to same spin orientations, as by this the position of minimal spin energy is approached. Minimal spin energy can only be achieved when particles overlap by same spin directions. So after some time all interacting particles will turn either to electrons or all to positrons. Because of the omnipresent and radiated photons the interacting particles can be in a wide range like a complete galaxy. Thus there might be galaxies consisting of matter and other galaxies consisting of antimatter. But the knowledge of the exact ratio in the universe is not relevant, because in any way matter and antimatter are identical particles.
That bound particles have always the same spin orientation can easily be tested by magnetic fields. For example an electron pair decays to two electrons. But you have to take into account that both electrons are emitted in opposite directions. So if you don't change the direction of the magnetic field the two electrons are deflected to opposite directions, which might lead to the erroneous thinking that the two electrons are a positron and a electron.