Question

why our atmosphere appears so dark

Answer 1

The daytime sky is blue because light from the nearby Sun hits molecules in the Earth's atmosphere and scatters off in all directions. ... At night, when that part of Earth is facing away from the Sun, space looks black because there is no nearby bright source of light, like the Sun, to be scattered

Answer 2

The atmosphere is because it is thick. It approximately contains 2,500,000,000,000,000,000,000,000 (2.5 X 10^25) atoms per cubic centimeter of air. Therefore it is thick.
Hope this helps....