Cosmic microwave background (CMB)

The existence of this is quite mind blowing.

It is basically perfectly black-body radiation, with a very faintly by measurable anisotropy (slightly less or slightly more some regions) due to quantum fluctuations of the early universe.

To make sense of it, you can think of the universe as the expanding raisin bread model, but it expands faster than light (thus the existence of the cosmological event horizon), so we are still receiving light form the middle, not the borders.

The ansiotropies of CMB are likely the best astronomical compass we will ever have, as it is the thing with the least proper motion.