The Big Bang theory does not specifically address how the first particle formed from nothing. Instead, it describes the early stages and subsequent expansion of the universe from an extremely hot and dense state. The theory is based on observational and theoretical evidence, such as the cosmic microwave background radiation and the abundance of light elements, as well as the mathematical framework of general relativity.
According to the Big Bang theory, the universe underwent a rapid expansion and cooling from a highly compressed and hot state. At extremely high energies, the fundamental forces of nature, including gravity, electromagnetism, and the strong and weak nuclear forces, were unified into a single force. As the universe expanded and cooled, these forces began to separate into distinct entities.
In the early universe, particle-antiparticle pairs could be spontaneously created from the available energy due to the uncertainty principle of quantum mechanics. These pairs could also annihilate each other, returning their energy back to the universe. However, during an epoch known as cosmic inflation, a period of exponential expansion in the early universe, the rapid expansion caused some regions to become isolated from each other, effectively freezing out the annihilation process. This freezing out allowed some particles to persist and form the building blocks of matter.
As the universe continued to expand and cool, the energy levels dropped, and particles such as quarks and electrons began to form. Through various interactions and processes, these particles eventually combined to form protons, neutrons, and other subatomic particles. As the universe further expanded and cooled, the conditions became suitable for the formation of atoms, allowing for the emergence of neutral atoms like hydrogen and helium.
It is important to note that the Big Bang theory describes the evolution of the universe from a fraction of a second after the initial singularity, but it does not provide a complete understanding of what occurred at the exact moment of the singularity or how the universe originated from "nothing." The question of how the universe began or how the first particle formed remains an active area of research and speculation in cosmology and theoretical physics.