If there is no cloud in the sky at all, a clear blue sky can be seen. But why does the sky actually have the color blue?
The sun radiates white light to Earth. But that white light is actually a combination of all kinds of light colors. You only see those colors when the light is refracted, just like with a rainbow.
To reach Earth, the white sunlight must first pass through the atmosphere. The atmosphere is a layer of gas surrounding the Earth. That gas consists of very small particles: the gas molecules.
The light collides with those molecules. As a result, the light is sent in all directions. We call this scattering. But not every color is scattered equally. Like yellow and red. Those colors are hardly scattered, and therefore radiate less to Earth.
The blue light scatters a lot more. That’s because the wavelength of this light is shorter. Due to the scattering, the blue light spreads over the entire atmosphere. Because the blue light reaches the earth from all sides, we see the sky as blue.
There are no gas molecules in the universe, so the light is not scattered in space. Therefore, astronauts see a black sky with a white sun. Pretty boring don’t you think? Blue is a lot more beautiful!