The most abundant element in the universe is hydrogen. Comprising about 75% of the elemental mass of the universe, hydrogen is a fundamental building block in the cosmos. It plays a crucial role in the process of stellar nucleosynthesis, where hydrogen atoms fuse under extreme temperature and pressure in stars to form heavier elements like helium, carbon, and oxygen. As the simplest and lightest element, hydrogen exists mainly in stars and gas giant planets. Its abundance is a key factor in the formation and evolution of galaxies.