Final answer:
The Big Bang Theory describes the universe's expansion from a highly compressed state 13.8 billion years ago. As the universe expanded and cooled, particles formed, leading to the creation of matter and the CMB radiation, which supports this theory.
Step-by-step explanation:
The Big Bang Theory is a central concept in cosmology that describes the birth of the universe. About 13.8 billion years ago, all matter and energy in the observable universe were compressed into a minuscule volume. Contrary to the incomplete statement in the question, these contents were not drawing together but undergoing a massive and rapid expansion. This expansion is akin to an explosion of space itself rather than material exploding through space as in a conventional blast.
As the universe expanded, it cooled, leading to the formation of protons, neutrons, and later electrons. During these early stages, high-energy photons collided to create particles, and this cooling pattern allowed the universe to transition from a hot, dense state to a cooler one with a Swiss-cheese-like structure. Dense regions within this structure were the precursors to galaxies, as gravitational forces allowed matter to cluster. The cosmic microwave background (CMB) radiation, a relic radiation detected today, provides critical evidence for this theory, indicating the universe's age and the distribution of matter in its early state.
Today, we understand that normal matter makes up about 5% of the universe's total mass-energy content, with dark matter and dark energy accounting for the remainder. The theoretical development of the Big Bang has been attributed to scientists such as Lemaître, Alpher, and Gamow, with ongoing research continuing to refine our understanding of the universe's origins and composition.