The perception of outer space as black is primarily due to a combination of factors related to the absence of significant light sources and the scattering of light. Here are a few key reasons:
Lack of atmospheric scattering: On Earth, the atmosphere scatters sunlight, resulting in the blue color of the sky during the day. However, in space, there is an absence of atmosphere (or any significant amount of matter) to scatter sunlight. Consequently, when viewed from space, the direct sunlight doesn't scatter off anything and simply travels in a straight line, making space appear dark.
Vast distances and low light levels: The universe is incredibly vast, with vast distances between stars and galaxies. Light originating from distant stars and galaxies has to travel extremely long distances to reach our eyes or telescopes. As it travels across such immense distances, the light can get dimmer due to factors like absorption or scattering by interstellar dust or gas clouds. This can contribute to the overall darkness of space.
Background radiation: Space is filled with a faint background radiation called the cosmic microwave background radiation. This radiation is a relic of the early universe, originating from the Big Bang. It is at a very low energy level and is not visible to the human eye. Hence, it does not significantly contribute to the perceived color of space.
Contrast with celestial objects: Although space itself appears black, it often serves as a backdrop against which celestial objects like stars, galaxies, and planets can be observed. These objects emit or reflect light, allowing them to stand out against the dark background.
It's worth noting that space isn't completely devoid of light. Stars, galaxies, nebulae, and other celestial objects emit light. Additionally, specialized instruments like telescopes and satellites equipped with sensitive sensors can detect and capture light from distant sources that may not be visible to the naked eye.