The Big Bang theory is the prevailing scientific explanation for the origin of the universe. According to this theory, the universe began as an extremely hot and dense state approximately 13.8 billion years ago. However, it is important to note that the concept of a single atom as the origin of the Big Bang is not accurate.
The universe is thought to have originated from a singularity, which is a point of infinite density and temperature. This singularity is not a single atom but a state where the laws of physics, as we understand them, break down. It is difficult to describe what exactly existed before the Big Bang or how the singularity came into being, as our current understanding of physics breaks down at that point. The singularity represents the moment when the universe rapidly expanded and began to evolve into the complex and diverse cosmos we observe today.
It is worth noting that the Big Bang theory is based on extensive observational evidence, such as the cosmic microwave background radiation and the observed expansion of the universe. However, the precise details of the initial singularity and the exact nature of the universe's origin are still areas of active scientific research and investigation.