Explain the big bang theory?

Hint: In 1927, Georges Lemaître discovered that an expanding universe could be traced back in time to a single originating point, which he dubbed the "primeval atom." In 1929, Edwin Hubble confirmed that galaxies are indeed drifting apart by analyzing galactic redshifts; this is important observational evidence for an expanding universe.

Complete step-by-step solution:
The Big Bang theory is the dominant cosmological model of the observable universe from its beginnings to its subsequent large-scale evolution. The model describes how the universe expanded from a state of high density and temperature, and it provides a comprehensive explanation for a wide range of observed phenomena, such as the abundance of light elements, the cosmic microwave background (CMB) radiation, and large-scale structure.
Crucially, the theory is consistent with the Hubble–Lemaître law, which states that the farther away galaxies are, the faster they move away from Earth. Using known physics laws to extrapolate this cosmic expansion backward in time, the theory describes an increasingly concentrated cosmos preceded by a singularity in which space and time lose meaning (typically referred to as "the Big Bang singularity").Detailed measurements of the universe's expansion rate place the Big Bang singularity around 13.8 billion years ago, which is thus considered the universe's age.

Note: The Big Bang theory provides a comprehensive explanation for a wide range of observed phenomena, including light element abundances, the CMB, large-scale structure, and Hubble's law. Two major assumptions underpin the theory: the universality of physical laws and the cosmological principle. One of the underlying principles of the theory of relativity is the universality of physical laws.