Big Bang
The Big Bang theory describes the universe's origin from an extremely hot, dense state around 13.8 billion years ago, followed by rapid expansion and cooling that continues to this day.
During the first fraction of a second—an era called inflation—the universe expanded exponentially, smoothing and flattening space. After inflation ended, the universe consisted of a highly energetic plasma of particles and radiation.
Within minutes, protons and neutrons combined to form light elements (hydrogen, helium, deuterium, and small amounts of lithium) in a process known as Big Bang nucleosynthesis.
About 380,000 years after the Big Bang, the universe cooled enough for electrons and nuclei to combine into neutral atoms. This allowed photons to travel freely, producing the Cosmic Microwave Background—the oldest light we can observe.
Over time, gravity amplified slight density variations in the plasma, leading to the formation of stars, galaxies, and large-scale structures.
Key evidence supporting the Big Bang includes the expansion of space (observed via galaxy redshifts), the abundance of light elements, and the precise measurements of the Cosmic Microwave Background by missions such as COBE, WMAP, and Planck.
Modern cosmological models (ΛCDM) incorporate dark matter and dark energy, explaining current observations of cosmic acceleration, geometry, and composition.
Sources
APODs including "Big Bang"
BOOMERANG Images The Early Universe
3 May 2000
Drifting through the stratosphere above Antarctica in late 1998, the balloon-borne BOOMERANG telescope peered into the cosmos at millimeter wavelengths. The blotchy structures it detected are seen above in the sharpest yet picture of the universe at an early age, perhaps a mere 300,000 years old. The false-color image shows subtle fluctuations in the temperature of the hot plasma which filled the universe before expansion cooled the material, producing the familiar stars and galaxies. Dramatically, the size of the fluctuations in the BOOMERANG image has convinced many cosmologists that the universe contains just exactly enough matter and energy to be flat -- a powerful prediction of the popular theory of Inflation which describes the earliest moments of the Big Bang. A flat universe will expand forever and two light rays which start out parallel will never spread or intersect. But all is not solved (!) as the BOOMERANG results don't show clearly details confirming the suspected amounts of dark matter and the cosmological constant.