Later This site tells the story of the history of the universe. Click Earlier and Later to follow the story. The macrocosmos was filled with high energy radiation. Radiation is a mixture of electric and magnetic fields which grow and die as they move along, swapping energy between them, somewhat like the two wings of a strange bird which flaps first one wing and then the other, something like the picture we print here.
Some radiation has more energy than others.
In the young Universe the radiation mostly had very high energy. In radiation with high energy, the fields swap energy faster than in low energy radiation.