Sun lights all the eight planets by some amount of its rays. Yet why the solar system is dark???
Answers
If the entire solar system is illuminated by the sun, why is it dark in outer space?
Because light has to have something to light up, and space is mostly made of nothing at all. But if you are wondering why the night sky is dark, that’s an excellent question, and was known as Olber’s Paradox: if the universe is infinite, no matter where you look, there should be the surface of a star there. The entire sky should be blindingly white.
Einstein solved that one, with Edwin Hubble helping him quite a lot. Einstein’s general relativity seemed to show that the Universe was expanding, and Hubble’s observations showed that it was 13.2 billion years old (actually he figured a bit less, but our observations got better). So while the Universe might be infinite in space, it is not infinite in time. When you look at a patch of sky that is completely black, even in our best telescopes, you are looking at a patch where, in a manner of speaking, no stars have yet formed.
The whole picture is a bit more complicated. If you look further out in space, you also look further back in time, and in those dark patches, you are actually looking at the Big Bang. But the light from the original fireball has been “stretched” by the expansion of the Universe, so it now looks incredibly dark—it looks like the explosion only reached 2.7K, a temperature so low that you need some good laboratory equipment to achieve it here on Earth. The light is known as the “cosmic microwave background”.
You can observe it for yourself, if you have an old-fashioned TV. Turn it on and make sure it’s not tuned to any channel (the proverbial “war of the ants”). About 30% of the static you see comes from the afterglow of the Big Bang.