What is matter? What everything is made of, in plain words

Matter is anything that has mass and takes up space: the air, water, rock, food and living things around you, and the stars. All ordinary matter is made of atoms, and atoms are made of a few smaller particles: electrons, and protons and neutrons, which are in turn made of quarks. Light, heat and sound are not matter, though they carry energy. That split, between what things are made of and what they do, is what this note is about.

In short

• Matter is stuff with mass that fills space. Everything you can touch is built from atoms, and every atom from electrons and quarks.
• Nearly all the mass of an atom sits in its tiny nucleus, and most of a proton's mass comes from the energy binding its quarks, not from the quarks themselves.
• Ordinary matter, the kind made of atoms, makes up only about 5% of the universe. The rest is dark matter and dark energy, which nobody has yet identified.

What counts as matter, and what doesn't?

The everyday test has two parts: matter has mass, so it has weight in a gravitational field and resists being pushed, and it takes up space, so two pieces of it can't sit in the same place. A brick, a glass of water and the air in a balloon all pass. Air is easy to forget, but a balloon pumped full is measurably heavier than an empty one.

Some real things fail the test. Light has energy and can push on things, as solar sails show, but a particle of light, a photon, has no mass at rest, so it isn't counted as matter. Heat is not a substance either. In the late 1700s and early 1800s many scientists thought it was a fluid they called caloric, until experiments showed that heat is the motion of the particles in matter. Sound is a vibration traveling through matter, not matter itself. In each case the thing is a form of energy, moving through or between pieces of matter.

Physicists use the word a little more narrowly. In particle physics, matter means the particles called fermions, the quarks and leptons, which are the building blocks, as distinct from the particles that carry forces between them. In everyday use and in chemistry, matter is simply whatever has mass and volume.

What is matter made of, from a grain of salt down to quarks?

Every level of matter is made of smaller parts, and the steps are enormous. A single grain of table salt, a cube about 0.3 mm on a side, weighs roughly 0.06 milligrams. Salt is sodium chloride, with an average of 58.44 grams per mole of its formula units. Divide and multiply by Avogadro's number, 6.02 × 10²³, and the grain holds about 6 × 10¹⁷ sodium ions and as many chloride ions: roughly a billion billion particles in something you can barely see.

Level	Example	Typical size
Grain of salt	One crystal of sodium chloride	About 0.3 mm (3 × 10⁻⁴ m)
Ions in the crystal	Distance from a sodium ion to its chlorine neighbor	0.28 nanometers (2.8 × 10⁻¹⁰ m)
[[what-is-an-atom	An atom]]	A carbon atom
Nucleus	A carbon nucleus, 6 protons and 6 neutrons	About 5 femtometers (5 × 10⁻¹⁵ m)
Proton or neutron	Three quarks bound together	About 1.7 femtometers across
Quark or electron	The smallest known pieces	No size measured: smaller than about 10⁻¹⁸ m, as far as experiments can tell

According to CERN's summary of the Standard Model, the theory that describes these particles, all the stable matter in the universe is built from just three of them: the up quark, the down quark and the electron. Two ups and a down make a proton; one up and two downs make a neutron. Heavier quarks and leptons exist, but they decay within a tiny fraction of a second, and are seen in cosmic rays and particle colliders.

How many protons sit in an atom's nucleus decides which element it is, which is the organizing idea of the periodic table. Atoms join into molecules, molecules gather into solids, liquids and gases, and those make up everything else.

Is matter mostly empty space?

In one sense, yes. A carbon atom's six electrons weigh about 5.5 × 10⁻³⁰ kg in total, while the whole atom weighs about 2.0 × 10⁻²⁶ kg, so about 99.97% of an atom's mass sits in a nucleus tens of thousands of times smaller than the atom itself. Matter packed as tightly as a nucleus would have a density of about 2 × 10¹⁷ kg per cubic meter, so a teaspoon of it, 5 milliliters, would weigh about a billion tonnes. That is roughly what the inside of a neutron star is like.

In another sense, no. The space between the nucleus and the edge of the atom is not a vacuum with a few tiny electrons orbiting in it. Electrons behave as waves spread out through that space, so the atom is filled by its electron cloud. A table feels solid because the electron clouds of its atoms repel your hand's electron clouds, and because a rule of quantum physics, the Pauli exclusion principle, stops electrons from crowding into the same state. "Mostly empty" describes where the mass is, not whether the space is used.

Is mass the same thing as matter?

Not quite, and the difference is one of the more surprising facts in physics. Matter is the stuff; mass is a property it has. The two don't add up the way you'd expect. The three quarks in a proton account for only about 1% of its mass. The other 99% comes from the energy of the quarks' motion and of the force field that binds them, which appears as mass through Einstein's E = mc². Most of the weight of your body, in other words, is bound-up energy.

Mass and weight differ too. Mass is how much matter resists being accelerated, measured in kilograms, and it is the same on the Moon as on Earth. Weight is the pull of gravity on that mass, so an astronaut weighs about a sixth as much on the Moon while her mass doesn't change.

What forms can matter take?

The same matter can be a solid, a liquid, a gas or a plasma, depending on how fast its particles move and how tightly they hold on to each other. Water is ice, liquid and steam without its molecules changing at all. The states of matter note follows what happens to the particles at each step, and why it takes energy to move between them.

Matter also has a mirror image. Every particle has an antiparticle with the same mass and opposite charge: the electron has the positron, the proton the antiproton. When a particle meets its antiparticle they annihilate, and their mass turns entirely into energy. CERN notes that particles and antiparticles are always made in pairs, so the Big Bang should have made equal amounts of each, yet the universe we see is almost all matter. The leading idea is that some unknown process left about one extra particle of matter for every billion antiparticles, and everything that exists is that leftover.

What is dark matter?

Ordinary matter, the kind made of atoms, makes up only about 5% of the content of the universe, according to NASA. About 27% is dark matter and about 68% is dark energy. Dark matter is called that because it gives off, absorbs and reflects no light. It is known only from its gravity: galaxies spin and clusters of galaxies hold together as if they contained far more mass than their stars and gas, and that unseen mass bends the light of galaxies behind it.

Still argued

Most astronomers accept that dark matter is real and is some kind of particle not yet detected, because several independent kinds of evidence point the same way. But no experiment has caught a dark matter particle, and candidates such as WIMPs and axions remain unconfirmed. A minority of researchers argue that gravity itself behaves differently over galactic distances, which would remove the need for dark matter in some cases. Which is right is not settled. The dark matter note sets out the evidence.

Check yourself

• Without looking back, explain why light is not counted as matter, even though it carries energy.
• In what sense is an atom mostly empty space, and in what sense is that picture misleading?
• Where does most of the mass of a proton come from, if not from its quarks?

Sources

• The Standard Model — CERN
• Antimatter — CERN
• Dark Matter — NASA Science
• Electron mass — NIST, CODATA 2022
• Proton — Wikipedia
• Matter — Wikipedia