The reason the Universe can be glowing all around us but still look dark comes down to the physics of light in an expanding universe. When space expands, and the distance between objects grows, the light passing between those things gets stretched out, shifting the light to lower frequencies on the electromagnetic spectrum.

For visible light, lower frequencies correspond to redder colours, so this effect is called “redshift”. You can think of it like a Doppler shift – the same kind of effect that’s responsible for a siren dropping to a lower tone when an ambulance speeds away from you

The stranger physics comes in when you ask, what are those things whose light has travelled for that long? The Big Bang theory says that the Universe 13.8 billion years ago was a hot, dense inferno, in which all of space was filled with glowing-hot plasma, rippling and churning like the surface o...