The concept of time, as we understand it, breaks down at the moment of the Big Bang. The Big Bang is the current prevailing scientific model for the origin and early development of the universe. According to this model, the universe began as a singularity—a point of infinite density and temperature—approximately 13.8 billion years ago.
Since the singularity is a point of extreme conditions, it is not meaningful to talk about the existence of atoms or any other particles as we know them before the Big Bang. The universe as we know it, including the formation of atoms and other fundamental particles, emerged as the universe expanded and cooled down in the aftermath of the Big Bang.
Shortly after the Big Bang, the universe was extremely hot and dense, composed primarily of elementary particles such as quarks, leptons, and gauge bosons. As the universe expanded and cooled, these particles combined to form protons, neutrons, and electrons, which later assembled into atoms. The process of nucleosynthesis, which occurred a few minutes after the Big Bang, resulted in the formation of the lightest elements like hydrogen and helium.
Therefore, according to our current understanding, the formation of atoms and the subsequent development of matter occurred after the Big Bang, not before it. The Big Bang marked the beginning of the universe as we know it, including the fundamental building blocks of matter.