Astronomers do not explain the creation of energy. They assume it for lack of any way to explain its existence. Even astronomers don't know everything!

The Big Bang theory states that the Universe suddenly came into existence a long time ago with the sudden appearance of an unimaginably huge concentration of pure energy at a point in a superspace of which we know nothing. This point expanded into the four-dimensional space that contains the Universe that we know. Measurements on the cosmological microwave background radiation that appeared 380,000 years later have indicated that this happened 13.7 billion years ago. According to this theory, this concentration of pure energy at a single point instantly began to expand, thereby producing the expanding universe. Its contents consisted of pure electromagnetic energy with a spectrum indicative of a black body with a temperature of billions or even trillions of degrees. There existed no matter at all in the Universe at that time. As expansion progressed and the temperature fell, matter condensed out of the energy via Einstein's E = mc² somewhat analogous to the way that codling steam condenses into water or cooling water freezes into ice.