For a long time, the "biogenic theory" of petroleum origin has dominated mainstream thinking, positing that petroleum forms from the burial and thermal evolution of ancient microbial, plant, and animal remains in sedimentary environments. However, traditional theories cannot fully explain how complex biological macromolecules precisely crack into relatively simple hydrocarbon small molecules over geological time scales, and controversies remain regarding the energy sources and kinetic mechanisms of the cracking process. Integrating the core physical principle that cosmic expansion induces atomic expansion[1][3], we can construct a novel petroleum generation mechanism: petroleum is a product of sedimentary organic matter derived from microbial, plant, and animal remains, which undergoes gradual cracking and recombination of molecular structures under the sustained action of atomic expansion driven by cosmic expansion over hundreds of millions of years, ultimately transforming from complex macromolecules into hydrocarbon small molecules.