The universe is vast, but one element overwhelmingly dominates the cosmic inventory: hydrogen. Hydrogen is the simplest and most fundamental element, typically consisting of a single proton orbited by a single electron. This simplicity makes it the foundational building block for nearly all other matter. Its presence governs the life cycles of stars, the formation of galaxies, and the overall structure of the cosmos. The answer to “how much hydrogen is in the universe” is an overwhelmingly large percentage of all ordinary matter.
Quantifying Cosmic Abundance
Hydrogen constitutes the vast majority of all ordinary, or baryonic, matter in the universe. Baryonic matter is everything made of protons and neutrons, distinct from dark matter and dark energy. When measuring the elemental mass of the universe, hydrogen accounts for approximately 75% of this baryonic matter.
If the count is based on the sheer number of individual atoms, hydrogen’s dominance is even more pronounced, making up over 90% of all atoms in the cosmos. The next most abundant element, helium, contributes about 23% to 25% by mass. Less than 2% of the universe’s elemental mass is composed of all the heavier elements, such as oxygen, carbon, and iron, which are the products of stellar evolution.
The Origin Story: Hydrogen’s Birth
The immense abundance of hydrogen is a direct consequence of the universe’s earliest moments, explained by Big Bang Nucleosynthesis (BBN). Following the Big Bang, the universe was an extremely hot and dense plasma. As it expanded and cooled within the first few minutes, the temperature dropped sufficiently for protons and neutrons to form stable atomic nuclei.
The initial conditions strongly favored the formation of hydrogen, which is a single proton. Neutrons began to decay into protons, setting the initial ratio of protons to neutrons at about seven to one. This ratio ensured far more protons remained as single hydrogen nuclei than combined to form heavier elements like helium.
The nuclear fusion process lasted only about 200 seconds before the universe cooled too much for the reactions to continue. During this brief window, virtually all the neutrons combined with protons to form helium and trace amounts of other light elements. The overwhelming majority of original protons remained unreacted, leaving a primordial soup of approximately 75% hydrogen and 25% helium by mass.
Where the Vast Majority of Hydrogen Resides
The hydrogen measured today is distributed across several massive cosmic reservoirs, but most of it is not found inside stars. The largest fraction of baryonic matter resides in the Intergalactic Medium (IGM), the extremely tenuous gas that fills the immense voids between galaxies. This IGM is a diffuse web of mostly hydrogen and helium, holding the bulk of the universe’s ordinary matter that has not yet collapsed into galaxies.
Within a galaxy like the Milky Way, hydrogen is concentrated in the Interstellar Medium (ISM), the gas and dust found in the space between star systems. This galactic gas is primarily composed of hydrogen, with different phases including cold, neutral atomic hydrogen (H I) and hot, ionized hydrogen (H II). Stars are continually forming out of the densest pockets of this hydrogen gas, known as molecular clouds.
Inside stars, hydrogen acts as the nuclear fuel, being consumed through fusion to create helium and heavier elements. While stars contain enormous amounts of hydrogen, the cumulative hydrogen mass locked within stars is less than the total mass spread across the IGM and ISM.
Techniques Used to Measure Cosmic Hydrogen
To quantify and map this pervasive but often invisible cosmic hydrogen, scientists rely on specialized observational techniques. One highly effective method is the analysis of absorption lines in the light from distant quasars. As the quasar light travels across billions of light-years, it passes through clouds of intergalactic hydrogen gas. Each cloud absorbs specific wavelengths, creating an intricate pattern of absorption lines called the Lyman-alpha forest, which reveals the density and distribution of hydrogen.
A different technique maps the cold, neutral atomic hydrogen within galaxies, which emits a distinct radio signal. This signal is the 21-centimeter line, a specific wavelength produced when the electron in a neutral hydrogen atom spontaneously flips its spin state. Although this spin-flip transition is rare for a single atom, the sheer volume of neutral hydrogen ensures a detectable signal. Radio telescopes detect this 21-centimeter radiation, allowing astronomers to map the structure, mass, and dynamics of hydrogen gas clouds and the spiral arms of galaxies.