Do you know why we see blue color sky and 
not any other color?
That’s because Atmosphere has small particles 
which scatter blue light and hence we see
blue color sky.
Red light is not usually scattered as it has 
longer wavelength when compared to the blue
light and that's the reason to use red light 
in danger signs as it travels long distance
without getting scattered even by fog or smoke.
If there was no atmosphere then there wouldn't 
have been any scattering due to no particles
present and hence the sky would’ve looked 
dark.
That’s the reason why space looks dark in 
colour.
