The Big Bang:

Its a common agreement among the cosmologes that the Big Bang was  about 13,7 billion. years ago. This moment was the starting point of creating the Univers.  The starting point  was properly what the cosmologes calls a singularity. This singularity contains a small point without any extension, but with a very high density of energy and mass (some common with black holes). This singularity was mainly constructed by  quantum foam, which means the lowest stage of protons and neutrons (elemantary particles). This is the basic elements of the atoms kernel.
According to the astrophysics, quantum foam at this stage has a remarkable power to "turn up and disappear" from nothing.  When the Universe was created, it was the singularity that extended vigorous from a small point (compared in size smaller than an atom), to something much larger. This vigorous extension is what we usually calls The Big Bang. It was no cosmic explosion, but a very powerful extension from a singularity.
Some wise believer think that the singularity was created by God or an almighty creator, but the fact is that the singularity  has a strange power to create this small point from nothing. This means that the singularity "probably" was created casual without any "outward reason".
Its therefor a wide agreement among the astrophysics and cosmologs, that the stage before the singlarity was nothing, absolute nothing.
Singularity to present time:


On the other side its usual to name the extension as the Big Bang, since the extension was almost like a powerful fiery (something like a "cosmic primordial soup that exploded"). This was the first stage on the development of the Univers. It was at this moment space, time and mass was created at the first time. After this the evolution went trough a short, but outmost dramatic phase with extremely powerful extension (called inflasion). It was in this phase the first elements (hydrogen and helium) was created. This period had a duration of at least 380 000 years, after the Big Bang.

From zero to inflasion period:


After the inflasion period the Univers continued to extend in a more normal rate. The further development used almost 200 mill. years to create the first stars (look below), and additional 750 - 800 mill. years to create the first galaxies. After this the Univers has continued to develop itself to the Univers we are knowing today.

Big Bang expansion: