How did our universe begin?
The universe, as we know it today, began as a singularity, even smaller than a single atom. It was an infinitely dense with all the matter we see around us today, squeezed together at a searing point at an extremely high temperature in space and time. With our current understanding of the fundamental laws of physics, as described by theories like general relativity and quantum mechanics, around 13.8 billion years ago, an abrupt, rapid expansion occurred, which we now name "the Big Bang".
This event (the Big Bang) classifies as the beginning of the universe.
The runway process of expansion is called "inflation". With inflation and the cooling of the universe, subatomic parts like protons, neutrons, and electrons began to create, which later emerged together to create an atom.
Likewise, over billions of years, gravity caused all distinct matter to clump together and create stars, galaxies, and many more celestial objects. As the cosmic bodies evolved, new, diverse structures started developing and grew into the more complex forms we see today.