The question of how much of the universe is hydrogen is central to understanding the cosmos, as this element is the fundamental building block of all visible matter. Hydrogen (H) is the simplest atom, consisting of a single proton and typically a single electron. Its overwhelming presence is a direct consequence of the universe’s earliest moments, making it the most abundant element known. Exploring its cosmic prevalence, formation history, and distribution provides a clear picture of the material composition of galaxies and the vast spaces between them.
Quantifying Cosmic Abundance
Hydrogen abundance is typically measured in two distinct ways. By mass, hydrogen accounts for approximately 75% of all ordinary, or baryonic, matter. This establishes it as the dominant element, with helium constituting nearly all the remaining mass.
However, if we count the elements by the sheer number of individual atoms, hydrogen’s share increases dramatically to about 90%. This difference occurs because hydrogen is the lightest element. Since a hydrogen atom contains only one proton, while a helium atom contains two protons and two neutrons, hydrogen contributes far less mass per individual particle.
This dominance is observed consistently across distant galaxies and gas clouds, indicating a universal chemical uniformity for the lightest elements. This proportion is a crucial constraint for cosmological models.
The Origin Story: Hydrogen Formation
Hydrogen’s abundance is rooted in the conditions of the early universe during Big Bang Nucleosynthesis (BBN). Within the first few minutes after the Big Bang, the universe rapidly expanded and cooled, allowing fundamental particles to combine into protons and neutrons. The proton, the nucleus of common hydrogen, was the first stable composite particle to form.
Only when the universe cooled sufficiently, approximately three minutes after the Big Bang, could nuclear fusion begin, combining protons and neutrons. This brief BBN epoch was not long enough to create elements heavier than lithium, halting quickly due to the rapid expansion and cooling of the cosmos.
During this window, approximately 75% of the baryonic mass remained as individual protons (hydrogen nuclei), while nearly all the remaining 25% was fused into helium nuclei. The constraints of BBN ensured that hydrogen would remain the dominant element, as the formation of heavier elements requires the intense conditions found only inside stars.
Hydrogen’s Primary Residence
The hydrogen created during the Big Bang is spread across the cosmos in several forms, serving as the raw material for all cosmic structures. Within galaxies, hydrogen is the primary component of the Interstellar Medium (ISM), the gas and dust found between stars.
This galactic hydrogen exists in two main states: neutral and ionized. Neutral hydrogen (HI) is found in cold, diffuse clouds and is detectable through its unique 21-centimeter radio emission line. Ionized hydrogen (HII) is a hot plasma of protons and free electrons found in bright nebulae, where intense ultraviolet radiation from massive young stars strips the electrons from the hydrogen atoms.
Beyond the confines of galaxies, a significant fraction of hydrogen resides in the Intergalactic Medium (IGM), the tenuous gas filling the space between galaxies. This IGM is mostly hydrogen that has been ionized by radiation from distant quasars and the earliest stars. The IGM acts as a reservoir of material, slowly feeding the growth of galactic structures over cosmic time.
The Cosmic Census: Comparing Hydrogen to Other Elements
A complete cosmic census requires placing hydrogen’s abundance within the context of the universe’s total composition. All atoms—hydrogen, helium, and trace amounts of heavier elements (metals)—collectively form baryonic matter. This visible, ordinary matter makes up stars, planets, and gas clouds.
Baryonic matter, however, is only a small component of the total mass-energy content of the universe. Cosmological observations indicate that baryonic matter constitutes only about 4.9% of the universe’s total density. Therefore, the 75% hydrogen by mass figure applies only to this small fraction of visible material.
The remaining 95% of the universe is composed of two components that do not interact with light: Dark Matter and Dark Energy. Dark Matter accounts for approximately 26.8% of the total density, while Dark Energy makes up roughly 68.3%. While hydrogen dominates the elemental composition of the visible universe, the vast majority of the cosmos consists of non-atomic components.