The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the Universe to cool and resulted in its present continuously expanding state.
Imagining the big bang as an explosion originating at a specific location in space, represents a gross violation of the cosmological principle. This principle states that the universe has no preferred location, nor a preferred direction. You can think of the cosmological principle as nothing more than the Copernican debunking of geocentrism driven to its ultimate consequence: from a cosmological perspective our place in the universe is not special, and neither is any other places. Therefore the big bang did not happen at a particular place. It happened everywhere. The big bang represents a temporal singularity rather than a singularity in space.
In most popularized science sources, BBT is often described with something like "The universe came into being due to the explosion of a point in which all matter was concentrated." Not surprisingly, this is probably the standard impression which most people have of the theory. Occasionally, one even hears "In the beginning, there was nothing, which exploded."
There are several misconceptions hidden in these statements:
The BBT is not about the origin of the universe. Rather, its primary focus is the development of the universe over time.
BBT does not imply that the universe was ever point-like.
The origin of the universe was not an explosion of matter into already existing space.
While these early-Universe scenarios are still untested, it has become clear that the answers to some of the most pressing and fundamental questions facing cosmology today must involve events that took place during the first 0.01 sec of the history of the Universe. These fundamental questions include: the origin of the matterantimatter asymmetry, the nature of the dark matter, the origin of the smoothness and flatness of the Universe, the origin of the density inhomogeneities that initiated structure formation, the origin of the expansion, and even the ultimate fate of the Universe.
Since light travels at a finite speed, astronomers observing distant objects are looking into the past. Most of the stars that are visible to the naked eye in the night sky are 10 to 100 light years away. Thus, we see them as they were 10 to 100 years ago. We observe Andromeda, the nearest big galaxy, as it was about 2.5 million years ago. Astronomers observing distant galaxies with the Hubble Space Telescope can see them as they were only a few billion years after the Big Bang.
The CMB radiation was emitted 13.7 billion years ago, only a few hundred thousand years after the Big Bang, long before stars or galaxies ever existed. Thus, by studying the detailed physical properties of the radiation, we can learn about conditions in the universe on very large scales at very early times, since the radiation we see today has traveled over such a large distance.
At first the many independent ways of measuring the age of the universe gave a wide variety of estimates, ranging up to twenty billion years or more, and astronomers argued over whose results were best. But around the end of the twentieth century, the answers began to converge — agreeing on an age in the range of twelve to fifteen billion years. That still left room for vigorous debate. As old tools were refined and yet more new ones were devised, the answer was narrowed down to 13.7 billion years, give or take a few hundred million.
The protons and neutrons that form the nuclei of every atom today are relic droplets of that primordial sea, tiny subatomic prison cells in which quarks thrash back and forth, chained forever. Even in violent collisions, when the quarks seem on the verge of breaking out, new "walls" form to keep them confined. Although many physicists have tried, no one has ever witnessed a solitary quark drifting all alone through a particle detector.
During those early moments, matter was an ultrahot, superdense brew of particles called quarks and gluons rushing hither and thither and crashing willy-nilly into one another. A sprinkling of electrons, photons and other light elementary particles seasoned the soup. This mixture had a temperature in the trillions of degrees, more than 100,000 times hotter than the sun's core.
Hoyle's triumph in explaining how most elements could be created in stellar interiors fell outside the theory in which elements were created at the very start. It was interpreted as favoring a rival theory. And Hoyle did favor a rival theory, which he had played a large part in inventing and developing. In this theory the universe had always looked much as it does now. There never had been a "big bang"—a phrase that Hoyle invented in 1950, intending the nickname as pejorative.
The Big Bang model was a natural outcome of Einstein's General Relativity as applied to a homogeneous universe. However, in 1917, the idea that the universe was expanding was thought to be absurd. So Einstein invented the cosmological constant as a term in his General Relativity theory that allowed for a static universe. In 1929, Edwin Hubble announced that his observations of galaxies outside our own Milky Way showed that they were systematically moving away from us with a speed that was proportional to their distance from us. The more distant the galaxy, the faster it was receding from us. The universe was expanding after all, just as General Relativity originally predicted! Hubble observed that the light from a given galaxy was shifted further toward the red end of the light spectrum the further that galaxy was from our galaxy.
The simplest description of the theory would be something like: "In the distant past, the universe was very dense and hot; since then it has expanded, becoming less dense and cooler." The word "expanded" should not be taken to mean that matter flies apart -- rather, it refers to the idea that space itself is becoming larger. Common analogies used to describe this phenomenon are the surface of a balloon (with galaxies represented by dots or coins attached to the surface) or baking bread (with galaxies represented by raisins in the expanding dough).