The Big Bang theory is the idea that the universe was born through the rapid expansion of subatomic particles and is not in fact infinite, but is rapidly expanding. This expansion, which is called red shift, also seems to be accelerating. Originally before 1996, scientists assumed that the expansion of the universe would slow over time. Gravity would create a force that would pull inward on the galaxies supposedly causing them the expansion to decelerate over time. That theory has since been disproven with the discovery of an unknown substance that has been dubbed the name dark energy. Not to mistake the ability of naming a concept as the ability to understand that concept, no one has any current understanding as to whether dark energy is some kind of dynamical fluid unbeknownst to physics, or whether it is just a blatant property of empty space, is currently unknown. Cosmic background radiation was discovered in 1964, which together with the subtle expansion of the universe, makes the Big Bang theory the accepted theory among scientists.

Nasa's website holds a lot of information for The Big Bang, some of which is explained here. For further details be sure to check out their site!

The Big Bang, contrary to its name, was not an explosion. It was the rapid expansion of a very small amount of space in a very small amount of time. In this small, hot, dense environment, energy would form and exist for only split seconds. Pairs of quarks would form from gluons and instantaneously manifest and annihilate each other. It was hot in this environment that matter and energy were practically equivalent.

Inexplicably, one billion and one matter particles would form for every one billion antimatter particles; and at this point, the universe has expanded to nearly a billion kilometers in diameter. By this point, several new laws came into effect in the universe including the strong and weak nuclear forces, the electromagnetic force, and gravity. The universes expansion to this size has dramatically decreased the temperature and quarks begin forming hadrons, such as protons and neutrons. There are actually a lot of combinations of quarks that can form many variations of hadrons, except only very few of them remain stable.

At this point, the universe is one second old and becomes cold enough to allow neutrons to decay into either protons or an isotope of hydrogen called deuterium, thus creating the first element, hydrogen. Within the next couple of minutes, the universe cooled even further. More hydrogen atoms were created from various hadrons and electrons creating an electrically neutral environment and better environmental stability.

This period is called the Opaque Era or the “Dark Age” because there were no stars and the hydrogen gases were so dense that they didn’t allow any visible light to pass through. After millions of years, the condensed hydrogen gas formed stars and galaxies and their radiation dissolved the stable hydrogen gas into plasma. This plasma made the universe permeable allowing visible light to pass through space. Finally, visible light becomes actually visible for the very first time (although there likely were not any complex organisms around to witness it).

Now, the beginning of the universe, the Big Bang itself, is where physics takes a very questionable turn. Before the rapid expansion of space itself, when everything was considered nothing, the tools scientists are currently equipped with are suddenly rendered unusable as the very concepts of physics, time, and gravity do not appear to follow any conventional rules. In order to understand this better, scientists are trying to discover a new theory; one that unifies Einsteins’ Theory of Relativity and quantum mechanics. Although that does arouse brand new questions, such as:

• Were there universes before the one we inhabit right now?
• Is this the one and only universe to ever exist?
• What caused the Big Bang or did it just naturally occur based on laws we have yet to discover?

We might never be able to accurately answer these questions, but scientists do know that this is where the universe originally began; creating very minute particles that eventually gave the building blocks to everything that exists today: galaxies, stars, planets, even you and me. As we are constructed from the same building blocks that construct enormous galaxies, we are not separate from the universe. We are essentially the universe's way of experiencing itself, so let us keep asking questions about our origins until there are no more questions to ask.