Why is space dark even though the Sun shines?

Light only becomes visible when it reaches the eye. The blue daytime sky arises because air molecules scatter short-wavelength sunlight in all directions: scattered light arrives from the entire sky dome, outshining the stars. Above the atmosphere, that medium is missing. There the Sun burns without any filter, but the space next to it remains black, because no particles deflect its light toward the observer. That is why images of the lunar surface show dazzling terrain under a completely black sky, with no transition at the horizon.

Dark does not mean energy-poor in space. Radiation is simply strictly directional: a surface facing the Sun heats up strongly, while the opposite side cools at the same time, because there is no air to distribute the heat. Shadows are also hard, since without scattered light there are no soft transitions; edges look knife-cut. For cameras this means extreme contrasts; for spacesuits, satellites, and solar sails, a construction made of insulating foils, coatings, and radiators that must withstand both extremes at once.

There remains the question of why the countless stars do not light up the background. Astronomy struggled precisely with that in the so-called Olbers' paradox: in a universe of infinite age filled uniformly with stars, every direction of view should glow. It does not, because the universe has a finite age and so light reaches us only from a limited surrounding region, and because the expansion of space shifts the radiation from distant sources into the infrared and microwave range. What remains is a residual glow that no eye sees, and the impression of total darkness.