According to current theories in particle physics, it is believed that all subatomic particles have corresponding antiparticles. Antiparticles have the same mass as their corresponding particles but have opposite electric charge and other quantum numbers.
Antiparticles were first predicted by theoretical physicist Paul Dirac in the late 1920s as a consequence of his relativistic equation describing the behavior of electrons, now known as the Dirac equation. The Dirac equation predicted the existence of both positive and negative energy solutions, leading to the interpretation that negative energy solutions correspond to antiparticles.
Many antiparticles have been discovered and observed experimentally, including the positron (the antiparticle of the electron), antiprotons, and antineutrons. In fact, the discovery of the antiproton in 1955 and the antineutron in 1956 were important milestones in particle physics.
Now, regarding the specific question of why we don't commonly encounter anti-protons and anti-neutrons in everyday matter, it is primarily due to their scarcity and the effects of cosmic ray interactions.
Antiparticles are generally rare in our observable universe because, during the early stages of the universe's evolution, there was a process called baryogenesis that led to an asymmetry in the production of matter and antimatter. This asymmetry is still not fully understood, and it resulted in a predominance of matter particles, such as protons and neutrons, compared to their antiparticles.
Additionally, when high-energy cosmic rays, such as protons and nuclei, collide with particles in the atmosphere, they can create particle-antiparticle pairs. However, these antiparticles are typically short-lived and quickly annihilate when they encounter their corresponding particles, such as protons and neutrons in the atmosphere. Therefore, the antimatter produced in these interactions is not stable enough to form antimatter nuclei like anti-protons or anti-neutrons.
Nevertheless, scientists can create and study antiprotons and antineutrons in particle accelerators and high-energy experiments. These experiments provide valuable insights into the properties of antimatter and help refine our understanding of particle physics.