It is believed that the universe originated around 15 billion years ago in the form of dense, hot blob of gas expanding rapidly outward. During that period, the universe contained only hydrogen and a small amount of helium. No stars and planets existed at that time. When the universe was some 100 million years ago, then only the first stars apparently began to form out of hydrogen. This is also the process of origin of Sun about 4.49 billion years ago.
Big Bang Theory
Georges Lemaitre (1927) had proposed a model for origin of the universe which became the Big Bang theory. The Big Bang theory explains the origin of universe, with the key idea that the universe is expanding. In the past, the universe was denser and hotter. All of space was contained in a single point, which is considered the inception of the universe. After the initial expansion, the universe freezes that allowed the creation of subatomic particles, consisting of protons, neutrons, and electrons. The majority of atoms formed were hydrogen, along with helium and traces of lithium. Huge clouds of these primal elements later fused through gravity to form stars and galaxies. The massive elements were synthesized either within stars or during supernova. Hence, the Big Bang theory does not explain the initial condition of the universe, but instead it describes the general evolution of the universe.
The Big Bang theory is based on Albert Einstein’s theory of general relativity. This also takes into consideration the homogeneity and isotropy of space. It was Edwin Hubble (1929), who discovered that the distances to remote galaxies were firmly correlated with their redshifts. According to Hubble, all observable areas of the universe are receding from each other.
Once there were two theories for explaining the expansion of universe, the Big Bang and the Steady State theory. But in 1964, with the discovery of the cosmic microwave background radiation, the Big Bang theory was confirmed.
In 1992, the launching of the Cosmic Background Explorer (COBE) satellite showed that 99.7% of the energy of the universe was released within the first year of its evolution. This confirmed the Big Bang theory, which proposes that the universe originated from a single violent explosion (which gives the name big bang) of a very minute amount of matter of high density and temperature.