The Big Bang theory is the prevailing cosmological model for the world in the earliest known phases through its succeeding large-scale evolution. If the known laws of physics are extrapolated into the maximum density regime, the result is a singularity which is typically related to the Big Bang. Physicists are undecided whether this means the universe started from a singularity, or current knowledge is inadequate to describe the universe at that time. Detailed measurements of the growth rate of the universe place the Big Bang at around 13.8 billion years ago, which can be thus considered the era of the world. Following the first expansion, the universe cooled sufficiently to allow the creation of subatomic particles, and after simple atoms. Giant clouds of those primordial elements afterward coalesced through gravity in halos of dark matter, finally forming the stars and galaxies visible now.
Since Georges Lemaître first noticed in 1927 that an expanding universe could be traced back in time to an coming single stage, scientists have built on his notion of cosmic expansion. The scientific community has been once divided between supporters of two distinct theories, the Big Bang, and the Steady State theory, but a wide array of empirical evidence has strongly preferred the Big Bang which is currently universally accepted. In 1929, from analysis of galactic redshifts, Edwin Hubble concluded that galaxies are drifting apart; this is essential observational evidence by the hypothesis of an expanding world. Back in 1964, the cosmic microwave background radiation has been detected, which was critical evidence in favor of the Big Bang model, since that theory predicted the existence of background radiation throughout the world before it was discovered. The known physical laws of nature could be used to calculate the characteristics of the world in detail back in time to an initial state of intense density and