One of the strange things about our expanding universe is that we can actually look in any direction and see light from the Big Bang. If you think of the Big Bang as happening at a single point in space, this is pretty confusing, but it makes more sense if you understand that the Big Bang happened everywhere in space at the same time, and it’s space itself that has expanded ever since.
The light we can detect from the Big Bang emerged out of an expanding ‘foggy’ fireball when the universe became transparent about 13.8 billion years ago, just 380,000 years after the explosion itself. Thanks to its long journey across the ever-growing cosmos, it’s now been stretched so that we detect it not as visible light, but as microwave radio waves. We can only see this ‘background radiation’ from parts of the universe that are at just the right distance for the radiation to be reaching us right now – so that’s why it seems to come from 13.8 billion light years away, regardless of the direction we look in.
Answered by Giles Sparrow