In the traditional Big Bang model, the universe began from an initial singularity, a state of extreme density and temperature.
The term describes the limit reached when the equations of classical general relativity are extrapolated backward in time. Density, temperature, and spacetime curvature become mathematically unbounded in that description. The universe then expanded and cooled, allowing particles, nuclei, atoms, stars, and galaxies to form.
However, a singularity is not necessarily a confirmed physical object or a literal point that scientists have observed. It may indicate that general relativity is incomplete under extreme conditions and must be replaced or supplemented by a theory of quantum gravity.
This cosmic singularity should not be confused with the singularity predicted inside an idealized black hole. Both arise in classical equations, but they describe different spacetime situations and remain subjects of active theoretical research.