Baryonic matter
Baryonic matter is the scientific term for ordinary matter: everything composed of atoms, and therefore of protons, neutrons, and electrons. This is the matter we are familiar with—the matter that makes up stars, planets, oceans, rocks, and living beings. Despite its apparent ubiquity in our daily lives, it accounts for only about 5% of the total content of the universe.
Why this name?
In particle physics, baryons are particles composed of three quarks. The best-known are the proton and the neutron, which make up atomic nuclei. The term “baryonic matter” therefore refers, by extension, to all matter composed of these nuclei, as opposed to dark matter (the nature of which is unknown) and dark energy.
5% of the universe
The observable universe contains approximately 68% dark energy, 27% dark matter, and only 5% baryonic matter. Of this 5%, a large portion is in the form of hot gas in galaxy clusters or in filaments in the intergalactic medium—relatively little is found in stars and planets. In other words, everything we can see directly—stars, galaxies, nebulae—represents only a tiny fraction of the universe’s content.
The Missing Baryonic Matter
For a long time, astronomers were unable to locate all of the baryonic matter predicted by the cosmological model. This is the problem of missing baryons. Recent observations have shown that a large portion of it exists as hot, diffuse gas in the cosmic filaments connecting galaxy clusters— too tenuous and too hot to be easily detectable, but definitely present.
Did you know?
Your body is made up of baryonic matter. The atoms that make up your body—carbon, oxygen, hydrogen, nitrogen—were all forged in stars that disappeared billions of years ago. Baryonic matter is both the most familiar and the rarest of the universe’s components.
Frequently Asked Questions
Does baryonic matter include electrons?
Electrons are not baryons (they are leptons), but they are included in the concept of ordinary matter or baryonic matter in the broad sense. Their mass is negligible compared to that of protons and neutrons, but they are essential to the structure of atoms.
What is the difference between baryonic matter and dark matter?
Baryonic matter is composed of protons and neutrons, interacts with light, and is detectable. Dark matter does not interact with light; its nature is unknown, and it can only be detected through its gravitational effects.
Why is baryonic matter so rare in the universe?
According to the standard cosmological model, primordial nucleosynthesis produced a slight asymmetry between matter and antimatter. Matter slightly dominated, giving rise to all of today’s baryonic matter. The rest of the universe is dominated by dark matter and dark energy, the nature of which remains a mystery.