When you look at the sky and see the darkness behind the stars, do you wonder about the nature of this darkness? I remind myself that what my eyes see as darkness is actually the primordial light that is coming to us from all directions. This primordial light was there even before the birth of a single star in the universe: invisible to our eye but visible to a radio telescope.
Looking a faint star through a telescope, we look back in time. In the moment of our looking, our eyes absorb the photon that was created in the star millions of years ago. The star might have been dead by now. The last photon it emitted toward us is might still be traveling in space. Perhaps, there will be no one on this earth to absorb that photon and witness the death of the star.
Had universe been existing from eternity, there would be no darkness in the night sky. If you would have pointed a dark patch to an astronomer in one direction, he would have said that we have to look further back in time with a more powerful telescope and we would find a star there. Our universe is not like this at all. The best telescopes have looked farthest in space and back in time till they find no stars there – they find a sphere beyond which there is total darkness. Yet, to some aliens that can see radio waves, this primordial darkness would be luminous. They would see it as the afterglow of the big bang – an event that created all that we see around us and all that we do not see as well.