Short answers, pulled from the story.
The Big Bang is a physical theory describing how the universe expanded from an initial state of high density and temperature. It explains the abundance of light elements, the cosmic microwave background radiation, the redshift of galaxies, and the large-scale structure of the universe.
Detailed measurements of the universe's expansion rate place the initial singularity at an estimated 13.787 billion years ago, which is considered the age of the universe. This age agrees with estimates from the oldest stars and from Type Ia supernovae.
Alexander Friedmann derived the equations of an expanding universe in 1922, and Georges Lemaitre proposed in 1931 that the universe emerged from a primeval atom. Edwin Hubble's 1929 observations with Milton Humason established that galaxies recede in proportion to their distance, now known as Hubble's law.
Astronomer Fred Hoyle coined the term during a March 1949 BBC Third Programme broadcast while contrasting it with his own steady-state model. The name did not catch on until the 1970s, and Hoyle denied it was meant as a pejorative.
The main evidence consists of four pillars: the expansion of the universe shown by galaxy redshifts and Hubble's law, the cosmic microwave background discovered in 1964, the relative abundances of light elements from Big Bang nucleosynthesis, and the formation and distribution of galaxies and large-scale structures.
In Big Bang cosmology, luminous matter makes up less than 5% of the universe's density, dark matter accounts for 27%, and dark energy makes up the remaining 68%. Dark energy drives the accelerating expansion observed in Type Ia supernovae, while dark matter is inferred from gravitational effects but has never been detected in a laboratory.