Parallel universes, or multiverses, are primarily speculative concepts rooted in theoretical physics, with varying definitions and classifications proposed by experts like Max Tegmark. While some ideas about multiverses are based on established theories, such as String Theory, they remain largely unproven. The existence of parallel universes raises questions about the nature of reality and the limits of our understanding of the universe. Current discussions emphasize the speculative nature of these theories, highlighting the need for further exploration and evidence. Overall, the debate continues on whether parallel universes are a tangible reality or merely theoretical constructs.