The sky appears to be dark to an astronaut because scattering of light at such altitudes is very small.
1. Scattering of light is only possible if there is any obstacle (object/particle) in the path of light.
2. When the light ray falls on the small particle of the atmosphere, the path of the ray deviates and rays spread out randomly.
3. As there is no atmosphere at a very high altitude, the sky appears to be black to the astronaut. This is because there is no scattering of light.
Hence, the given statement is true.