BIG BANG theory was states that the universe was born about fifteen to twenty billion years ago when infinitely dense, single point underwent a tremendous explosion. Sub-atomic particles formed shortly after the explosion and then hunled outward into space. The particles brought together by gravitational sources, forming atoms. Atoms then turn combined to produced planet, starts, galaxies, and other space objects.
In 1917, Dutch astronomer Willem de Sitter showed how Albert Einstein Calculation in his theory of relativity could be used to describe expanding universe. In 1922, Russian Mathematician Alexander Friedmann gave an exact solution for an expanding universe. Georges-Henri Lemaitre a Belgian astronomer and Jesuit Priest also gave contribution on this theory. By mid 1960 this theory had become widely accepted to explain how the universe begin. Recent study came in 1992, when NASA Cosmic Background Explorer (COBE) looked fifteen billion years into space (the same as looking fifteen billion years into the past. As a student with science background I do believe on this theory but as a human being I have great trust on GOD. Even British physicist and mathematician Stephen Hawking has questioned whether the universe has any beginning at all in terms of space time.
Source: Phillis Engelbert an Diane L Dupuis, The Handy Space Answer Books, Visible Ink Press, 1998