In the beginning universe was cold, sea of fundamental particles. The earlier universe was smaller, denser and had no structure. Structure in the universe evolved hierarchly (small to big) Cluster of galaxies have only been around for 7-8 billion years. There is a boring time before star formation (dark ages) It takes a couple million years from atoms --> star. It goes from the dark ages to a bright ear, where everything for ionized. In the early universe nuclear fusion occurs everywhere. The universe starts off really dense and hot, as it expands it gets cooler. Where you have something ionized (scattered electrons) light bounces off them and change direction. This caused radiation to bounce around everywhere (coupling) This coupling caused by the furious colliding electrons, protons, neutrons and photons causes so much heat that you can have nuclear fusion.