What event marks the origin of the universe according to Big Bang theory?

The event that marks the origin of the universe according to the Big Bang theory is characterized by the expansion of an initial hot, dense state. This theory suggests that the universe began as a singularity—a point of infinite density and temperature—approximately 13.8 billion years ago. At the moment of the Big Bang, this singularity underwent a rapid expansion.

This expansion is not just an explosion in space; rather, it represents the expansion of space itself, with all matter and energy coming into existence. As the universe expanded and cooled, fundamental particles formed, eventually leading to the creation of atoms, stars, galaxies, and the large-scale structure of the universe we observe today.

Understanding this initial condition is crucial to grasping the subsequent evolution of the universe, including the formation of galaxies and cosmic structures. This foundational event of expansion initiates the timeline of cosmic history, setting the stage for everything that follows.