The phrase "subatomic realm" refers to the extremely tiny world that exists within atoms. It describes the study of particles, such as protons, neutrons, and electrons, which are the building blocks of matter. Full definition
The realm is described by former Ant - Man Dr. Hank Pym as an inescapable subatomic realm «where all concepts of time and space are irrelevant.» (cbr.com)
When physicists investigated the subatomic realm, however, they discovered that the principle of least action is just a limiting case of the much more subtle and sophisticated path integral principle, which is the basis of quantum mechanics, as Richard Feynman showed in the 1940s. (firstthings.com)
The succession of scientific discoveries and revolutions in physics over the last 500 years have led to fundamental paradigm shifts (Kuhn, The Structure of Scientific Revolutions, 1962) in the way that science conceives of the reality of this universe both on the cosmological scale and in the subatomic realm, and perhaps even beyond this universe. (faith.org.uk)