Matter

Matter (also called substance or material) is the general term for anything that has mass and occupies space. In physics and chemistry, matter is what we see and touch: water, air, rock, and living bodies. Matter is fundamentally composed of atoms, which consist of protons, neutrons, and electrons.

The term matter does not have a single definition accepted by all scientists. Some define it as anything with rest mass; others limit it to things made of fermions such as electrons and quarks. For this reason, photons are generally not counted as matter, even though energy and mass can be converted into one another.

Structure

Everyday matter can be broken down into levels. The broadest level divides things into mixtures and pure substances. Pure substances are either elements or chemical compounds. An element consists of one type of atom, while a chemical compound is made of two or more types of atoms bonded together. Inside the atom, the nucleus—made of protons and neutrons—is at the center, with electrons orbiting around it. Protons and neutrons are themselves composed of quarks held together by the strong force.

States of matter

Matter exists in different states depending on temperature and pressure. The common ones are solid, liquid, and gas. A fourth state is plasma, in which atoms lose their electrons and become an ionized gas; most stars are plasma. There are also special states produced at very low temperatures, such as the Bose–Einstein condensate.

Mass, energy, and antimatter

Special relativity theory shows that mass and energy are closely related, as expressed in E = mc². This means matter can be destroyed and converted to energy while conserving total energy. Everything has a counterpart called antimatter, with identical properties but opposite charge. When matter and antimatter meet, they annihilate each other and release radiant energy.

Dark matter

Cosmology indicates that ordinary matter—atoms and their constituents—makes up only a small fraction of what exists in the universe. The rest is divided between dark matter and dark energy. Dark matter cannot be seen directly by light, but its effect on gravity is measured through the motion of galaxies.