The Big Bang is the origin of the universe, occurring approximately 13.7 billion years ago. It began as a point of nearly zero volume and tremendous density. Then this point started stretching outward in all directions, not expanding within space but causing the expansion of space itself.
The first period of time immediately after the Big Bang is known as the Planck epoch, which occurred during the first 10-43 seconds after it. Little is known about this period, because our current physical theories cannot probe smaller timescales than this. It is thought that all the four fundamental forces — strong nuclear, weak nuclear, electromagnetism, and gravity — were unified at this point, serving as one superforce. Scientists are working on physical theories to help describe this epoch. By the end of the Planck era, the force of gravity separated from the other three, creating gravity and the so-called electronuclear force.
After the Planck epoch was the grand unification epoch, occurring 10-43 to 10-35 seconds after the Big Bang. The universe was smaller than a quark (a type of subatomic particle) with temperatures higher than 1027 K. This is about 1012 times more energetic than collision points inside the largest particle accelerators. As the universe expanded and cooled, and the electronuclear force broke apart into its constituents: the strong nuclear force, the weak nuclear force, and electromagnetism. By the end of the grand unification epoch, the universe was about the size of a proton.
The grand unification epoch was followed by the inflationary epoch, during which the universe grew by a factor of at least 1026, and possibly much larger. The inflationary epoch lasted only about 10-32 seconds, but during this time, the universe grew from the size of a proton to the size of a grapefruit or larger. Its volume increased by a factor of at least 1078. The universe expanded many times faster than the speed of light, explained by noting that the space itself was expanding, even though nothing within space broke the universal speed limit.
After the inflationary epoch, the universe continued to expand, until it became what it is today — a behemoth at least 92 billion light years in size, and maybe much more.