Step-by-step explanation:
Different scientist gave different theories about universe.
According to Big Bang theory, scientists assumes that universe was about 13.8 billion years ago, universe was centered at a point and its temperature of over a trillion degrees. Due to really high temperature it took place an explosion. In this process a tremendous amount of energy was released and it also created different galaxies like Milky way, Andromeda, and Whirlpool galaxy. Which was the beginning of our universe. Big Bang was the most common theory of the universe.