The universe is a dynamic, evolving place. It is governed by cycles of matter and energy, an intricate series of physical processes in which the chemical elements are formed and destroyed, and passed back and forth between stars and diffuse clouds.
Big Bang Theory
According to the Big Bang theory, all current and past matter in the Universe came into existence at the same time, roughly 13.8 billion years ago. At this point, all matter had been compressed into a very small ball of infinite density and intense heat known as a Singularity. Suddenly, the Singularity began to expand, resulting in a big bang and the birth of the universe as we know it.
Arguments supporting the theory
In the 1920s, a Belgian priest named Georges Lemaître proposed the big bang theory, claiming that the universe began with a single primordial atom. The idea received a major boost from Edwin Hubble's observations that galaxies are speeding away from us in all directions. Edwin Hubble, an American astronomer, made an intriguing and significant discovery in the late 1920s. Hubble made observations that he interpreted as indicating that distant stars and galaxies are receding in all directions from Earth. Furthermore, recession velocities increase in proportion to distance, a discovery confirmed by numerous and repeated measurements since Hubble's time. These discoveries imply that the universe is expanding. Certain conclusions can be drawn from Hubble's theory of an expanding universe. One possibility is that the universe was once more condensed. This deduction led to the hypothesis that all of the currently observed matter and energy in the universe were originally condensed in a very small and infinitely hot mass.
As well as Arno Penzias and Robert Wilson's discovery of cosmic microwave radiation in the 1960s, which they interpreted as echoes of the big bang.
The expansion of the universe means more space between galaxies. Hoyle's concept of steady state was an alternative. It assumed that the universe was roughly the same at all times. However, as more evidence about the expanding universe becomes available, the scientific community now favours the expanding universe argument.