The Big Bang
The Universe is expanding. All the Stars and Galaxies are moving away from us. Whichever direction is space we look we see everything being flung out from us. The further away things are the faster they are moving, and things at the same distance from us are moving at the same speed. Looking at the Universe like this, it seems that way back when, things were much closer together than they are now. All in the same place in fact. It seems that this time, when everything was in the same place, was in fact the beginning of the Universe. Before the moment of creation we can only speculate, but we can we believe, trace back the history of the Universe right back to the moment just after the birth of the Universe, that moment of birth that we call the Big Bang.
According to the standard theory, our universe sprang into existence as "singularity" around 13.7 billion years ago. About this “singularity”, we know essentially nothing. All the laws of physics break down at the singularity, so we can't even be sure what it is. If we were to call it a “nothing”, that would signify as much. There are many Singularities – one at the centre of each black hole. The Singularity in question though gave birth to the Universe. What caused it? We don't know.
After its birth, it apparently inflated, expanded and cooled, going from very, very small and very, very hot, gradually cooling, going through an era of opaqueness – the radiation dominated era – to an era of transparency – the matter dominated era, in which star formation and galaxy formation became possible. The Universe continued to expand and cool and continues to do so to this day
Space didn't exist prior to the Big Bang. Physicists have extended Einstein's Theory of General Relativity to include measurements of time and space. According to calculations, time and space had a finite beginning that corresponded to the origin of matter and energy." The singularity didn't appear in space; rather, space began inside of the singularity. Prior to the singularity, nothing existed, not space, time, matter, or energy – nothing. We don't know where it came from, what caused it or what it was made of.
Galaxies appear to be moving away from us at speeds proportional to their distance - "Hubble's Law," This observation supports the expansion of the universe and suggests that the universe was once compact.
Third, if the universe was initially very, very hot as the Big Bang suggests, we should be able to find some remnant of this heat.. In fact we have found it in the form of the cosmic microwave background radiation. This has a signature temperature of about 3 degrees Kelvin – just above absolute zero.
The amounts of hydrogen, helium and other light elements are consistent with the Big Bang model.