The Big Bang Theory is a concept that provides an explanation how the universe was created. Without any religious considerations or any biased personal beliefs, this is the nearest theory we can get based on scientific observation and analysis.
The main idea of the Big Bang Theory is that the universe actually came into existence when a primordial start of elemental condition started to expand in the past. This condition is composed of very hot and dense initial state which catapulted in expanded direction that allowed the volume to increase and created the universe. The expansion was thought to be initially rapid and even instant. Some scientists even proposed that the “bang” catapulted space materials nearly or more than the speed of light. As the initial expansion lengthens and loses its momentum, the spreading out gradually decreases.
Georges Lemaitre is considered to be the proponent of the Theory as his presentation of assumptions supported mainly the idea. The model was basically patterned to the General Relativity Theory by Albert Einstein.