Matter is anything that has mass and occupies space. It is the stuff of the universe. Stars, planets, rocks, water, air, and living things are all made of matter. Matter is made of atoms, which are made of protons, neutrons, and electrons. Protons and neutrons are made of quarks. Quarks and electrons are fundamental particles, meaning they have no known internal structure. Matter can exist in different states: solid, liquid, gas, and plasma. Plasma is the most common state in the universe, found in stars and lightning.

Matter has several properties. It has mass, which resists acceleration. It has volume, which occupies space. It has density, which is mass per unit volume. It can undergo physical changes, like melting and boiling, and chemical changes, like burning and rusting. Matter is conserved in chemical reactions, but it can be converted into energy in nuclear reactions. The universe contains about 5 percent ordinary matter, 27 percent dark matter, and 68 percent dark energy. Dark matter and dark energy are not made of atoms, and their nature is unknown. Ordinary matter is the part we understand best.

States of matter

• Solid. Fixed shape and volume.
• Liquid. Fixed volume, variable shape.
• Gas. Variable shape and volume.
• Plasma. Ionized gas, most common in the universe.

Matter is not the same as mass. Mass is a property of matter. Matter is the physical substance that has mass and takes up space.