Why is space black?
Space looks black because it is almost empty, with no air to scatter sunlight, and because the universe is not infinitely old, so light from most distant stars has not reached us.
On Earth, the sky looks blue during the day because air molecules scatter sunlight in all directions, especially blue light. In space, there is almost no air, so sunlight travels in straight lines without scattering. Unless light enters your eyes directly from a star or reflects off an object, you see darkness.
Astronomers once asked a deeper question, known as Olbers' paradox: if the universe were infinite and full of stars, every direction should end at a star, and the night sky should be bright. The answer is that the universe has a finite age, about 13.8 billion years, so light from very distant stars has not had time to reach us. Also, the expansion of space stretches old light into wavelengths our eyes cannot see.
Not completely dark
Space is filled with the cosmic microwave background, the faint afterglow of the Big Bang, but it is invisible to human eyes. Telescopes sensitive to microwaves can detect it coming from every direction.
Did You Know?
Astronauts on the International Space Station see a black sky even in full sunlight, with the Sun appearing as an intensely bright disk.