It is currently unknown what events, if any, occurred prior to the universe reaching the size of one Planck unit. Therefore, it is theoretically possible that the universe did not have a definite beginning. However, the currently accepted and widely supported cosmological models, including the Big Bang theory, provide a comprehensive explanation for observed phenomena such as the cosmic microwave background radiation, the abundance of light elements, and large-scale structure. These models are also supported by a wide range of empirical evidence, including measurements of the expansion rate of the universe and observations of the redshifts of supernovae. The Big Bang theory predicts a uniform background radiation caused by high temperatures and densities in the distant past, which has been observed as the cosmic microwave background radiation. While there are aspects of the universe that are not yet fully explained by the Big Bang theory, it remains the most widely accepted explanation for the origin and evolution of the universe.
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