Simply put, it is the principle of light reflection. The reason why the daytime is not so obvious at night is that the brightness of the light in the daytime is strong in a large range, and the light outside the dust particles has a certain mixing and weakening effect on the reflected light of the particles themselves. Because of parallax, the human eye can't obviously observe the reflected light of the particles, and we can't easily see the dust. At night, there is less light reflected around, so dust particles reflect light more obviously and dust is easier to see.