The Origin of Particle Stability, Particle Generation in the Early Universe, the Nature of Neutron Decay, and the Structure of Dark Matter — A Comprehensive Review of Particle Physics in the First-Principle Unified Theory of Light Strings

Based on the First-Principle Unified Theory of Light Strings, this paper explains the origin of particle stability, particle generation in the early universe, the nature of neutron decay, and the structure of dark matter. It demonstrates that only electrons and protons are permanently stable elementary particles; neutrons are metastable proton–electron composites; dark matter is a symmetric stable composite of one proton and two electrons. All other particles are unstable excited states or transient fragments. This paper provides a unified, self-consistent, and paradox-free framework for particle physics and dark matter under light string theory.