The Big Bang theory is our best scientific explanation for how the universe began and evolved. According to this theory, the universe started from an extremely hot and dense state called a singularity about 13.8 billion years ago. Hubble's Law shows that distant galaxies are moving away from us, with velocity proportional to distance. The redshift formula helps us measure how fast objects are receding. As the universe expands, its temperature decreases inversely with the scale factor. Key evidence includes the cosmic microwave background radiation and the observed abundance of light elements.