Atheists do not believe in the existence of a god or gods, so they do not believe that the universe was created by a god. Instead, they believe that the universe came into existence through natural processes.

The Big Bang theory is the prevailing cosmological model for the observable universe from the earliest known periods through its subsequent large-scale evolution. The model describes how the universe expanded from an initial state of high density and high temperature, and offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, and large-scale structure.

According to the Big Bang model, the universe was born about 13.8 billion years ago in a state of extremely high density and temperature. This initial state is known as the Big Bang singularity. The universe has been expanding and cooling ever since.

The Big Bang theory does not explain what caused the Big Bang singularity, or what existed before the Big Bang. Some physicists believe that the universe may have existed in a previous state, and that the Big Bang was a phase transition from that state to the current universe. Others believe that the universe was created by a quantum fluctuation, or that it is a mathematical construct.

The origin of the universe is one of the most fundamental questions in physics. The Big Bang theory provides a good explanation for the evolution of the universe since the Big Bang, but it does not answer the question of what caused the Big Bang or what existed before the Big Bang. These are questions that physicists are still trying to answer.

Atheists do not have a single explanation for the existence of something from nothing. Some atheists believe that the universe may have always existed, while others believe that it may have come into existence through natural processes that we do not yet understand.

Here are some of the possible explanations that atheists have proposed for the origin of the universe:

* The universe may have always existed. This is the simplest explanation, but it is also the most difficult to prove.

* The universe may have been created by a quantum fluctuation. Quantum fluctuations are spontaneous events that occur in the vacuum of space. They are thought to be responsible for the creation of particles and antiparticles. It is possible that a quantum fluctuation created the universe.

* The universe may be a mathematical construct. Some physicists believe that the universe is a mathematical object that exists independently of physical reality. This theory is known as mathematical Platonism.

These are just some of the possible explanations for the origin of the universe. There is no single explanation that is universally accepted by atheists. However, all of these explanations are based on the scientific evidence that we have available, and they provide possible ways to explain how the universe came into existence.