The Big bang model is based on two assumptions. The first is Albert Einstein's General Theory of Relativity. It correctly describes the gravitational interaction of all matter. The second assumption, called the Cosmological Principles, states that an observer's view of the universe depends neither on the direction in which he looks nor his location. This principle applies only to the large-scale properties of the universe, but it does imply that the universe has no edge, so that the bigbang origin occurred not at a particular point in space but rather throughout space at the same time. These two assumptions make it possible to calculate the history of the cosmos after a certain epoch called the Planck Time. Scientists have yet to determine what prevailed before Planck Time.
In the first few minutes after the big bang with temperatre of at least 1 million degree, nuclear reactions started to create the basic structure of the gases hydrogen and helium. From about 380,000 to 1 millions years after the big bang universe cooled to about 3,000 degree centigrade, protons and electrons continued to make hydrogen atoms. Radiated energy of light and heat stemmed from the original expansion. Long process of formation of first stars started. At first new atoms joined a thin cloud of hot gases. The radiation that kept the atoms moving eventually weakened as millions of years passed. Atoms began to cluster together.