Why is hydrogen the most abundant element in nebulae?

Hydrogen is the most abundant element in the universe and, consequently, in most nebulae because it is a primordial element created during the big bang.1

Scientific observations and the big-bang theory establish that the universe began as a hot, dense state where nuclear reactions produced a specific initial chemical composition. This primordial matter consisted overwhelmingly of light elements, primarily hydrogen and helium. Because nebulae are composed of interstellar gas and material that reflects this original composition, they remain dominated by hydrogen.123

While stars can process and synthesize heavier elements through nuclear fusion, the vast majority of matter in the cosmos—including the gas clouds that form emission nebulae—retains this original, hydrogen-rich abundance. Even in planetary nebulae, which are enriched by material ejected from evolved stars, the typical composition remains approximately 1,000 times more hydrogen atoms than heavy elements.1345