What initial event do scientist hypothesize caused the universe to expand?

Scientists hypothesize that the universe expanded due to the Big Bang, a colossal explosion that occurred approximately 13.8 billion years ago. This event marked the beginning of space and time, leading to the rapid expansion of matter and energy from an extremely hot and dense state. As the universe expanded, it cooled, allowing for the formation of subatomic particles and eventually atoms, which led to the development of stars, galaxies, and other cosmic structures.