When the universe completed the Big Bang, matter continued to spread out in all directions, which is the expansion of the universe. As the universe expands, there is a slight difference in the density of matter, which causes matter in different parts of the universe to be concentrated by gravity towards a slightly higher density of matter nearby. Gravity increases and increases with the concentration of matter, and the greater the gravity, the more the matter becomes concentrated, and the concentration process cannot be stopped. Each place has a very large concentration area, perhaps even over hundreds of thousands of light years. With such a large area, the resulting structures are galaxies.
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Gravitational action in the concentration of matter