The world is made of atoms, and nearly everything you can see, touch, or breathe is built from a surprisingly short list of chemical elements. Zoom out further and the picture gets stranger: the entire observable universe is roughly 69% dark energy, 26% dark matter, and only about 5% the kind of ordinary matter that makes up stars, planets, oceans, and people. That familiar 5% is what we’ll focus on here, because it’s the stuff you actually interact with every day.
Atoms: Mostly Empty Space
Every solid object, every liquid, every gas is built from atoms. An atom has a dense nucleus of protons and neutrons surrounded by a cloud of electrons. What’s remarkable is the scale difference: the nucleus is about 100,000 times smaller than the atom itself. If the nucleus were a poppy seed sitting on a football field, the outer edge of the electron cloud would be near the goalposts. By volume, 99.999% of an atom is empty space. The reason a table feels solid when you press on it isn’t because matter is packed tight. It’s because the electric fields of neighboring atoms repel each other with enormous force.
What the Universe Is Made Of
Of the ordinary matter in the universe, hydrogen accounts for about 73% by mass and helium about 25%. Every other element, from carbon to iron to gold, makes up less than 2% combined. Stars are essentially giant balls of hydrogen and helium fusing together, and since stars vastly outnumber planets and moons, the universe’s composition reflects that.
Nearly all of this visible matter exists as plasma, a superheated state where electrons are stripped away from atoms. Plasma makes up 99.9% of the visible universe. The solid ground you stand on, the liquid water you drink, and the gas you breathe are extreme rarities on a cosmic scale.
What Planet Earth Is Made Of
Earth’s overall composition looks nothing like the universe at large. The planet is dominated by iron, oxygen, silicon, and magnesium because heavier elements sank toward the core during Earth’s formation while lighter ones floated outward. The core is mostly iron and nickel. The mantle, which makes up the bulk of Earth’s volume, is rich in silicon, oxygen, and magnesium bound together in dense mineral structures.
The thin outer crust you actually live on has its own distinct recipe. Ten elements account for 99.17% of its weight:
- Oxygen: 46.6%
- Silicon: 27.7%
- Aluminum: 8.1%
- Iron: 5.0%
- Calcium: 3.6%
- Sodium: 2.8%
- Potassium: 2.6%
- Magnesium: 2.1%
- Titanium: 0.4%
- Hydrogen: 0.1%
Oxygen and silicon together make up nearly three-quarters of the crust, which is why silicate minerals (combinations of silicon and oxygen with metals like aluminum, iron, or calcium) dominate virtually every rock you pick up. Gold, silver, and copper, the elements people prize most, are present in tiny fractions of a percent.
What the Air Is Made Of
Earth’s atmosphere is about 78% nitrogen and 21% oxygen by volume. Argon, an inert gas, fills most of the remaining 1%. Carbon dioxide, methane, and other greenhouse gases together account for roughly 0.04%, a sliver that nonetheless traps enough heat to keep the planet’s average temperature livable. Water vapor varies from nearly zero over deserts to about 4% in humid tropical air, which is why it’s usually excluded from “dry air” figures.
What the Oceans Are Made Of
Seawater is about 96.5% water molecules (hydrogen and oxygen) and 3.5% dissolved material. The two dominant dissolved ions are chloride and sodium, the ingredients of table salt. At a standard salinity of 35 parts per thousand, chloride accounts for about 19.4 grams per kilogram of seawater and sodium about 10.8 grams. Magnesium, sulfate, calcium, and potassium fill out most of the rest. These six ions make up well over 99% of the dissolved material in the ocean, which is why seawater tastes salty rather than, say, metallic or bitter.
What Living Things Are Made Of
Life reshuffles Earth’s elemental deck in a specific way. Four elements, oxygen, carbon, hydrogen, and nitrogen, account for 96.2% of the human body by weight. Oxygen alone makes up about 65%, largely because your body is roughly 60% water. Carbon is the backbone of every protein, fat, and strand of DNA. Hydrogen is abundant in water and organic molecules. Nitrogen is essential for amino acids, the building blocks of protein.
The remaining 3.8% includes calcium (mostly in bones and teeth), phosphorus (critical for energy transfer and DNA), potassium, sulfur, sodium, chlorine, and magnesium. Trace elements like iron, zinc, and copper each make up tiny fractions of your weight but play outsized roles. Iron, for instance, is what allows your red blood cells to carry oxygen.
The 5% and the 95%
Everything described above, every rock, ocean, cloud, and living cell, belongs to the 5% of the universe that’s ordinary atomic matter. The other 95% remains largely mysterious. Dark matter, making up about 26% of the universe’s total energy content, exerts gravity and holds galaxies together, but it doesn’t emit or absorb light. We know it’s there because galaxies rotate faster than they should based on their visible mass alone. Dark energy, the dominant 69%, appears to be driving the accelerating expansion of the universe. Its nature is one of the biggest open questions in physics.
So the world is made of atoms, built from a handful of common elements, arranged into an astonishing range of structures. But most of the universe is made of something else entirely, something we can detect but not yet explain.