Matter is the substance of which all material is made. That means objects which have mass. More specifically, they must have rest mass, which is a form of energy that matter has even when it is not moving (it has no kinetic energy), is extremely cold (it has no thermal energy), etc. Matter is a word that is sometimes used in varying ways in everyday life, whereas mass is a well-defined concept and quantity at least in physics. They are not the same thing, though they are related.[1]
Ordinary matter is made of tiny particles called atoms. The atoms have spaces between them and they move or vibrate all the time. The particles move faster and move further apart when heated, and then reverse when cooled.
Nearly all matter that may be experienced in everyday life is baryonic matter. This includes atoms of any sort, and gives those the property of mass. Non-baryonic matter, as implied by the name, is any sort of matter that is not composed mainly of baryons. This might include neutrinos and free electrons, dark matter, such as supersymmetric particles, axions, and black holes.
The very existence of baryons is a significant issue in cosmology. It is assumed that the Big Bang produced a state with equal amounts of baryons and antibaryons. How baryons came to outnumber their antiparticles is called baryogenesis.
Matter can be directly experienced through the senses. It has properties which can be measured, such as mass, volume, density, and qualitative properties such as taste, smell and colour, for instance.
All physical bodies in the universe are made of matter: galaxies, stars and planets, rocks, water and air. Living organisms like plants, animals and humans are also composed of matter.
In physics, the universe also contains things that aren't matter, including some elementary particles that have no rest mass. Photons (electromagnetic radiation such as light) are a familiar example.
In addition to its rest mass, matter can contain other forms of energy, which aren't matter but allow them to interact with each other by exchanging kinetic energy, heat, light, sound waves, etc.
Outside of the physical sciences, there can be many other things that aren't matter or energy. Just for example, emotions can be experienced or ideas can be had.
The structure and composition of matter is investigated by breaking matter into smaller and smaller pieces. Hence, living organisms are made up of cells. Cells are composed of molecules, which are sets of atoms bonded together. Each atom, in turn, is an assemblage of elementary particles.
Physicists also classify matter in a few broad categories, called states, with quite different properties:
A given amount of matter may change from one state to another depending on its temperature and pressure. On Earth, water can exist simultaneously in three states: solid (ice), liquid water (lakes, oceans) and gas (vapor or steam).