The Big Bang theory is the leading explanation for the origin and evolution of the universe, suggesting that it began as a singularity and expanded rapidly around 13.8 billion years ago. The Nebular Hypothesis is a widely accepted theory that explains the formation of our solar system, proposing that it formed from a giant cloud of gas and dust that collapsed under gravity. These theories provide a framework for understanding the structure, evolution, and diversity of the universe and our solar system.