The term "Big Bang" is used both in a narrow sense to refer to a point in time when the observed expansion of the universe (Hubble's law) began—measured to be 13.7 billion (13.7 × 109) years ago—and in a more general sense to refer to the prevailing cosmological paradigm explaining the origin and evolution of the universe.

One consequence of the Big Bang is that the conditions of today's universe are different from the conditions in the past or in the future. From this model, George Gamow in 1948 was able to predict the cosmic microwave background radiation (CMB). The CMB was discovered in the 1960s and served as a confirmation of the Big Bang theory over its chief rival, the steady state theory.

Overview of the Big Bang
Based on measurements of the expansion of the universe using Type Ia supernovae, measurements of the lumpiness of the cosmic microwave background, and measurements of the correlation function of galaxies, the universe has a measured age of 13.7 ± 0.2 billion years. The agreement of these three independent measurements is considered strong evidence for the so-called Lambda-CDM model that describes the detailed nature of the contents of the universe.

The early universe was filled homogeneously and isotropically with a incredibly high energy density and concomitantly huge temperatures and pressures. It expanded and cooled, going through phase transitions analogous to the condensation of steam or freezing of water as it cools, but related to elementary particles.

Approximately 10-35 seconds after the Planck epoch, a phase transition caused the universe to experience exponential growth during a period called cosmic inflation. After inflation stopped, the material components of the universe were in the form of a quark-gluon plasma (also including all other particles—and perhaps experimentally produced recently as a quark-gluon liquid[1]) in which the constituent particles were all moving relativistically. As the universe continued growing in size, the temperature dropped. At a certain temperature, by an as-yet-unknown transition called baryogenesis, the quarks and gluons combined into baryons such as protons and neutrons, somehow producing the observed asymmetry between matter and antimatter. Still lower temperatures led to further symmetry breaking phase transitions that put the forces of physics and elementary particles into their present form. Later, some protons and neutrons combined to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis. As the universe cooled, matter gradually stopped moving relativistically and its rest mass energy density came to gravitationally dominate that of radiation. After about 300,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is the cosmic microwave background.

Over time, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the universe. The three possible types are known as cold dark matter, hot dark matter, and baryonic matter. The best measurements available (from WMAP) show that the dominant form of matter in the universe is cold dark matter. The other two types of matter make up less than 20% of the matter in the universe.

The universe today appears to be dominated by a mysterious form of energy known as dark energy. Approximately 70% of the total energy density of today's universe is in this form. This component of the universe's composition is revealed by its property of causing the expansion of the universe to deviate from a linear velocity-distance relationship by causing spacetime to expand faster than expected at very large distances. Dark energy in its simplest formation takes the form of a cosmological constant term in Einstein's field equations of general relativity, but its composition is unknown and, more generally, the details of its equation of state and relationship with the standard model of particle physics continue to be investigated both observationally and theoretically.

All these observations are encapsulated in the Lambda-CDM model of cosmology, which is a mathematical model of the big bang with six free parameters. Mysteries appear as one looks closer to the beginning, when particle energies were higher than can yet be studied by experiment. There is no compelling physical model for the first 10-33 seconds of the universe, before the phase transition called for by grand unification theory. At the "first instant", Einstein's theory of gravity predicts a gravitational singularity where densities become infinite. To resolve this paradox, a theory of quantum gravity is needed. Understanding this period of the history of the universe is one of the greatest unsolved problems in physics.