We see the space between the galaxies as completely black, but it is filled with a very faint glow in the microwave spectrum. This glow is very cold, just under 3 degrees Kelvin and it fills the entire sky. You can hear it yourself as the static on the radio or television. Astronomers in the 1960’s were mystified by this phenomenon and at first they believed it to be interference from pigeon droppings in their antenna dish. Later they discovered that this mysterious radiation was the Cosmic Background Radiation and it was a direct leftover from the big bang. When the universe was very young and hot space was filled with almost uniform energy. however, it wasn’t completely smooth, there were tiny quantum fluctuations in the plasma and as the universe cooled and expanded these fluctuations multiplied exponentially and led to the clumping of galaxies we see today. In the image you can see the variations in temperature as different colours. You are looking at the echo of the Big Bang.