 Hydrogen , as atomic H , is the most abundant chemical element in the universe , making up 75 % of normal matter by mass and more than 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 . ) This element is found in great abundance in stars and gas giant planets . Molecular clouds of H2 are associated with star formation . Hydrogen plays a vital role in powering stars through the proton @-@ proton reaction and the CNO cycle nuclear fusion . 
