Space is mostly empty. That’s the first thing you have to wrap your head around if you want to understand what the universe is actually made of. But when you do run into "stuff"—stars, nebulae, the occasional planet—it isn’t a random mix of the periodic table. It’s overwhelmingly, almost boringly, consistent. If you took a giant cosmic blender to everything we can see, you’d find that hydrogen and helium make up about 98% of the entire observable mass of the universe.
Everything else? All the gold in your wedding ring, the oxygen you're breathing right now, the carbon in your DNA, and the silicon in your phone? That’s just a 2% rounding error.
The Big Two: Hydrogen and Helium Rule Everything
It’s easy to think of hydrogen as just "that gas that makes suns go," but its dominance is staggering. Hydrogen accounts for roughly 73% of the visible universe's mass. It’s the simplest atom. One proton. One electron. It was the first thing to congeal after the Big Bang, and it remains the primary fuel for every star currently burning in the sky.
Helium comes in second at about 25%. If you're doing the math, that leaves about 2% for literally everything else. Astronomers have a funny, slightly arrogant way of describing this. To a chemist, the periodic table is a masterpiece of nuance. To an astrophysicist, there are three things: Hydrogen, Helium, and "Metals." Anything heavier than helium is labeled a metal. Oxygen? Metal. Neon? Metal. Iron? Definitely metal.
This lopsided distribution isn't an accident. It's a direct fossil record of the Big Bang. During the first few minutes of existence, the universe was a hot, dense soup. It was basically a giant nuclear fusion reactor. This period, known as Big Bang Nucleosynthesis, lasted only about 20 minutes. It was long enough to fuse protons into helium nuclei but not long enough to cook up much else before the universe expanded and cooled too much for fusion to continue.
Why Oxygen is the "Best of the Rest"
After the big two, the numbers drop off a cliff. Oxygen is the third most abundant element, but it only makes up about 1% of the total mass. You might wonder why oxygen beat out things like carbon or nitrogen.
It comes down to stellar cooking. Most of the "heavy" elements (the 2% we talked about) are forged inside the bellies of stars. Through a process called the alpha process, stars fuse helium nuclei into heavier elements. Because an oxygen nucleus is essentially four helium nuclei stuck together, it's a very stable and "easy" element for a star to produce once it runs out of its initial hydrogen stash.
The Periodic Table’s Top Ten List
If we look at the universe by mass, the rankings stay pretty consistent.
- Hydrogen (~73%): The undisputed king.
- Helium (~25%): The perennial runner-up.
- Oxygen (~1%): The primary "impurity."
- Carbon (~0.5%): The backbone of life.
- Neon (~0.13%): Surprisingly common, though inert.
- Iron (~0.11%): The "star killer" that signals a supernova is coming.
- Nitrogen (~0.1%): Crucial for planetary atmospheres.
It’s worth noting that these numbers change depending on where you look. If you look at Earth, the list is totally different. Here, iron and oxygen dominate because the lighter gases—hydrogen and helium—are too light for our planet’s gravity to hold onto effectively in their gaseous forms. They drifted away into space eons ago, leaving us with the "heavy" dregs.
The Lithium Problem: A Cosmic Mystery
Science usually likes things to be neat. But there’s a nagging issue called the "Cosmic Lithium Problem." Based on our models of the Big Bang, we should see about three times more Lithium-7 in the oldest stars than we actually do.
Basically, our math says it should be there. Our telescopes say it isn't.
This isn't just a minor bookkeeping error. It suggests that either our understanding of nuclear physics in the early universe is slightly off, or there’s some "missing" physics—like dark matter interactions—that we haven't accounted for yet. Famous astrophysicists like Brian Fields have spent decades trying to reconcile this gap. It’s a reminder that even when we talk about the most abundant elements in the universe, we’re still working with an incomplete map.
👉 See also: Why 0 0 255 rgb Is the Most Powerful Blue You’ll Ever See
How We Actually Know This (Without Leaving Home)
You might be wondering how we can be so sure about the composition of a galaxy 10 billion light-years away. We use Spectroscopy.
Every element has a "fingerprint." When an atom gets excited by energy, its electrons jump around and emit light at very specific wavelengths. By passing starlight through a prism-like device called a spectrograph, we see a rainbow with dark lines missing. Those missing lines tell us exactly which elements the light passed through.
In 1868, Pierre Janssen and Norman Lockyer noticed a yellow line in the sun’s spectrum that didn’t match any known element on Earth. They named it Helium, after the Greek sun god Helios. We actually discovered the second most abundant element in the universe in space before we ever found it on our own planet.
The Iron Limit and the Death of Stars
Stars are great at making elements, but they have a hard ceiling: Iron.
Fusion releases energy. When you fuse hydrogen into helium, you get a massive payout of heat and light. This continues up the chain through carbon, neon, and oxygen. But fusing iron actually consumes energy rather than releasing it.
When a massive star develops an iron core, the engine stalls. The outward pressure stops. Gravity wins instantly, the star collapses in on itself, and—BOOM—a supernova. It’s only in the violent, energetic chaos of that explosion (and the collision of neutron stars) that the truly heavy elements like gold, silver, and uranium are born.
What This Means for Life and Technology
The fact that carbon and oxygen are in the top five isn't just a fun fact; it's why we exist. We are made of the most common "scraps" in the cosmos.
In terms of technology, our future is tied to these abundances. We are currently seeing a massive push toward Hydrogen Fuel Cell technology and fusion energy. Why? Because the fuel source is the most abundant thing in existence. If we can master the same process the stars use to fuse hydrogen, we essentially unlock an infinite energy bar.
On the flip side, we are constantly hunting for "Rare Earth" elements. They’re called rare for a reason. They sit at the bottom of the abundance list because the universe simply doesn't make much of them.
Actionable Insights for the Curious
If you want to dive deeper into how the building blocks of reality work, don't just read about them—look at them.
- Check the NASA APOD: The Astronomy Picture of the Day often features nebulae. Look for the "false color" images. Usually, Red represents Hydrogen, Green is Oxygen, and Blue is Sulfur. It’s a visual map of cosmic abundance.
- Follow the JWST: The James Webb Space Telescope is currently analyzing the atmospheres of exoplanets. It’s looking for the "Big Two" plus water vapor (hydrogen and oxygen). Any shift in these ratios could hint at alien life.
- Understand Your Tech: Realize that while the universe is 73% hydrogen, your smartphone relies on elements like Neodymium and Europium that are cosmically rare. This scarcity drives global economics and geopolitics.
- Watch the Lithium News: Keep an eye on new papers regarding "Primordial Abundances." If someone solves the Lithium Problem, it’ll be the biggest news in physics since the Higgs Boson.
The universe is a massive recycling project. The hydrogen in the water you drank this morning was likely formed 13.8 billion years ago. The oxygen was forged in a star that died long before the Earth was born. We aren't just looking at a list of gases; we're looking at a family tree.