For passengers flying at very high altitude, the sky appears dark. Why?
At very high altitude, scattering is not prominent.
The scattering of the light takes place because of the particles present in the atmosphere. The scattered light is what we see as the colour of the sky. At high altitude, due to the absence of the atmosphere, the scattering of light does not take place.
Hence, the sky appears dark to the passengers flying at high altitude.