The Big Bang theory is the cosmological model that describes the early development of the Universe. According to scientists, the Big Bang occurred approximately 13.789 billion years ago.  This is considered to be the age of the universe. The universe was extremely dense and hot at this time but rapidly started to expand. This cooled the universe sufficiently and allowed the energy to be converted into subatomic particles such as protons, neutrons and electrons.
The first element that was produced in the universe was hydrogen. After this, there were traces of helium and lithium. These elements formed giant clouds and later coalesced through gravity to form stars and galaxies. Other elements were then synthesized within stars or during supernovae.
The Big Bang theory is well tested and widely accepted. It offers an explanation for a wide range of observed phenomena such as, abundance of light elements, large scale structures, cosmic microwave background, and the Hubble diagram.
The Big Bang theory does not describe what happened in the Universe prior to the universe's expansion. Scientists have tried to model the initial expansion with accelerators. However, these accelerators have limited capabilities to probe into such high energy regimes.
 "Planck reveals an almost perfect universe". Planck.ESA . 2013-03-21. Retrieved 2013-03-21.