The moment of the Big Bang is considered by many scientists to be the beginning of time. There is some contention among scientists about when the Big Bang occurred, though estimates place it in the range of a minimum of 13 to 15 billion years ago. As of 2011, the Big Bang Theory is one of the most commonly accepted explanations of how the universe began.
A Tiny Point of Matter
Many scientists are of the opinion that, before the Big Bang, all of the energy in the universe was confined to one very small, very densely packed point. At this time, there was no matter or mass. There were no atoms or molecules. Then the point of energy exploded. As of May 2011, scientists have yet to identify the precise cause of the explosion, but the explosion itself has a commonly accepted name: the Big Bang.
For an amount of time much shorter than one billionth of a second immediately following the Big Bang, energy and heat were thrust outward from the explosion. The light and energy particles from the explosion travelled at a speed greater than the speed of light. Wherever the particles travelled, they created space. The space created by the Big Bang particles became the universe. However, at this time, the universe still had no mass particles.
Immediately following the inflation, the universe entered a superheated stage known as primordial soup. During the primordial soup stage, the heat resulting from the Big Bang was still so intense that particles could not combine to form atoms or mass. As a result, photons, electrons, protons, neutrons and other particles floated together. Light photons continually bounced off or attached themselves to electrons, giving the entire post Big Bang universe a constant glow.
Cooling and Expansion
As the light, and particles from the Big Bang continued to spread out, the heat between them became less concentrated. Consequently, after 300,000 years of existing as primordial soup, subatomic particles, such as electrons, neutrons and protons, which had cooled enough, began combining to create atoms. The first pieces of mass had come to be. Eventually, the atoms combined to create galaxies, stars, planets and all the matter of the universe.