As the universe expanded, that radiation didn't go away. It filled the universe, so it's still around today. When you make a map of the sky, you are essentially getting a snapshot of what the universe was like when it was very young. And that's extremely valuable if you want to try to test theories about how the universe evolved. By looking at this background radiation, researchers can see the temperature of the radiation in different parts of the sky.
Some regions are a little bit hotter than average and some regions are a little bit cooler.
It turns out that those variations exist because when the universe was very young and the background radiation was first formed, there were slight variations in how matter was distributed. Those density variations, it is thought, grew into the things we see around us today when we look at the universe. We think that galaxies formed from regions of slightly higher than average density. Because of gravity, those regions collapsed and formed galaxies and larger structures. So if you want to understand how our galaxy formed, the microwave background tells you the starting point.