At the moment when the universe completes the Big Bang, the matter is constantly flying away in all directions, which is the expansion of the universe. During the expansion of the universe, there is a slight difference in the density of various substances, which makes the parts of the universe in the role of gravity Concentrate towards a place with a slightly higher density of matter nearby. Gravity gradually increases and increases with the concentration of matter, while the greater the gravity, the more the matter becomes more concentrated. The area of material concentration is very large, and may even exceed hundreds of thousands of light years. Such a large area, the final structure is a galaxy. The text mainly says:. ['How galaxies are formed.', 'The physical process of universe expansion.', 'Why the Big Bang happened.', 'Gravity in the process of material concentration.']