This is due to the refraction of light. Sunlight has seven colors , blue light has a higher scattering capacity (excluding purple) and blue light is more abundant on Earth. As a result , light is reflected in various particles of the Earth's atmosphere and looks like a blue mantle on top. It is not a cover. So , in fact , there is no such thing as a sky. And the space is really dark. As a result , the sky looks blue from the moon or something else.