What theory suggests that the universe originated from an explosion of a small mass of matter at high density and temperature about 20 billion years ago?

The theory that suggests the universe originated from an explosion of a small mass of matter at high density and temperature around 20 billion years ago is the Big Bang theory. This foundation of modern cosmology posits that the universe began as an incredibly hot and dense point, which expanded rapidly. This expansion continues today, leading to the universe we observe, filled with galaxies, stars, and other cosmic structures.

The significance of the Big Bang theory lies in its ability to explain a variety of phenomena, including the cosmic microwave background radiation and the observed redshift of distant galaxies. As the universe expanded, it cooled, allowing particles to form atoms and eventually leading to the formation of stars and galaxies. The theory aligns with a wide array of observational evidence, solidifying its acceptance in the scientific community as the most comprehensive explanation for the origin of the universe.

In contrast, other theories, such as the Steady State theory, propose a different view about the universe's existence and development, suggesting it has no beginning or end, which directly contradicts the explosive origin proposed by the Big Bang theory. Cosmic inflation refers to a rapid expansion that occurred just after the Big Bang, while expansion theory is a broader term that does not specifically address the origins of the universe