Antiparticles are identical in mass to matter particles, but opposite to them in properties such as electrical charge. Few such particles exist in nature . . Presently, there is no evidence of antigalaxies. What ultimately seems decisive is the difficulty of imagining how matter and antimatter in the early universe could have become segregated into distinct regions. The principle is clear, however, and no physicist doubts it.
Antimatter can exist.
If they encountered ordinary matter, the massive annihilation reactions that result ought to be most noticeable. It ought to be, perhaps, but it is not. Astronomers have not spied any energy bursts anywhere in the sky that can be identified unequivocally as the result of matter-antimatter annihilation. This is a dilemma. That the moon and Venus are made of ordinary matter is clear from direct observations.