How is antimatter transient?
Antiparticles are intrinsically stable or unstable in the same way that their normal counterparts are: for instance, both electrons and positrons are stable, but muons (unstable particles) and their antiparticles both have a 2 microsecond lifetime. However, the universe is a hazardous place for an antiparticle, even if it’s intrinsically stable, since it’s likely to run into a normal particle and annihilate. Because antimatter never lasts very long once it’s created, it’s rare. Antineutrinos are an exception- they are quite common because they interact rarely.