Why does the sky appear dark instead of blue to an astronaut?

The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths i.e blue. When sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes and the sky appears blue. But there are no particles, hence no scattering of light in space. Therefore, the sky appear dark to an astronaut.