The Cosmic Microwave Background

In the mid 60s, the pinsius and wilson discovered the cosmic microwave background. This is the light that was produced when the first atoms were being formed a few hundred thousand years after the Big Bang. It's almost exactly the same temperature no matter what direction you look in. And this implies that the density of the universe - which we live in today - was also extremely uniform. But how well gravity does that job depends on how much dark matter there is.