 Cosmic microwave background anisotropy

There is a slight variation in temperature of CMB across the sky of the order of 200 microKelvin. It is small to the ~2.7 K average temperature, but it can be measured.

If the initial conditions of the Big Bang and the laws of physics were perfectly symmetric, then we could expect the Universe to just be one perfectly uniform boring soup.

But instead some asymetry made all the fun weird things we see today happen eventually, like galaxies and life.

And the cosmic microwave background serves as a way for us to look back in time to the early conditions of the universe, as it was set in stone as soon as the universe became transparent to this light during recombination.

Or if you want to get poetic, it is the closest we can ever get to listening to the original word of God when he setup the initial conditions of the universe.

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