The universe did not always exist. 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.