Why dos outer space apears black?

Answer 1

This same question was famously asked by 18th century astronomer, Heinrich Olbers; if the universe is static and filled with an infinite quantity of stars, any line of sight in any direction we might look in the night sky we should eventually end looking at a bright star or other illuminated object (despite the atmosphere's transparency, Rayleigh scattering of sunlight causes the daytime sky to appear bright, and have the blue color). There are many factors contributing to the apparent blackness of space. One is the transparency of the interstellar medium owing to the very tiny amount of matter in space (which is not already gathered gravitationally into stars, galaxies, planets, and so forth); another is the distribution of matter and the nature of matter itself - most matter in the universe is presumed not to interact electromagnetically, called "dark" - e.g., dark matter (otherwise it could heat up and glow); another is the age and expansion of the universe. The accepted scientific theories about the origin of the universe indicate there was a time when space was universally bright in all directions after the big bang - hot, dense, and opaque - but at a certain point ordinary matter condensed out of this plasma and the universe became transparent. The fairly directionally-uniform expansion of the universe brings other factors into play; since Hubble's law indicates that the further away galaxies are, the more quickly they recede away from an observer, there would be a theoretical limit at a certain distance where matter in an expanding universe would be redshifted into invisibility. Further, light from the big bang in an expanding universe would have its energy decreased into longer wavelengths not visible to the human eye - an idea supported by observations of a background cosmic radiation at microwave wavelengths. Other theories involving the nature of the universe itself may play a role, as might the finite age of the universe, the finite age of stars, and so forth.