section_path: Cosmic prevalence and distribution
section_name: Cosmic prevalence and distribution
paragraph_text: Cosmic prevalence and distribution
  
  Hydrogen, as atomic H, is the most abundant chemical element in the universe, making up 75% of normal matter by mass and >90% by number of atoms. Most of the mass of the universe, however, is not in the form of chemical-element type matter, but rather is postulated to occur as yet-undetected forms of mass such as dark matter and dark energy.
  
  Hydrogen is found in great abundance in stars and gas giant planets. Molecular clouds of H are associated with star formation. Hydrogen plays a vital role in powering stars through the proton-proton reaction in case of stars with very low to approximately 1 mass of the Sun and the CNO cycle of nuclear fusion in case of stars more massive than the Sun.