What is 'ordinary matter'?

Look around you. Look into the sky -- at night. Everything you see, the sun, the moon, distant galaxies, your body, your shoes, are all made of what we call ordinary atomic matter. It has another name which will be explained below: baryonic matter.

The Atomic Model

All atomic matter is made from the same basic building blocks: A central nucleus surrounded by a cloud of electrons.
In a little more detail, every atom consists of:

This model explains everything out there (except for dark matter, but hold your horses -- we'll get to that later). Here's how:

Galaxies, ignoring dark matter for the moment, are made of stars and dust. How are stars and dust made from the atomic building blocks? Well, stars are just big, hot collections of different combinations of different atoms. In particular, they start out as mostly hydrogen and helium, the simplest possible atoms. Hydrogen is defined as a nucleus with Z=1 proton. Most hydrogen has N=0, meaning no neutrons. So, hydrogen, the simplest element, is just a proton. In fact, we often write equations involving hydrogen in the sun using the symbol "p" meaning "proton" for hydrogen. If it has a neutron bound to the proton in the nucleus, we call it deuterium and its symbol is 2H. It can sometimes have even N=2 neutrons bound to the proton. That is called tritium and its symbol is 3H. A helium nucleus, He, has two Z=2 protons and either N=1 or N=2 neutrons, with the symbols 3He and 4He respectively.

Inside the stars, the nuclei of hydrogen and helium and whatever else might be around will fuse together to form bigger, more complex nuclei. In other words, if you take two nuclei with a nucleus Z=2 protons (helium), you can fuse them together to make a new nucleus with Z=4 protons (which happens to be called beryllium). And so on...

This process of fusing together nuclei with only a few protons and neutrons into nuclei with many protons and neutrons also happens to be the energy source which powers stars: thermonuclear fusion. By this stellar alchemy all of the other atoms of the universe were formed! Everything from the carbon (Z=6) in your body to the iron (Z=26) in the Earth to the sulfur (Z=16) in the atmosphere of Venus was formed somehow by a star. Even uranium with a whopping Z=92 protons and (usually) N=146 neutrons was made by a star (probably in a supernova explosion as opposed to regular stellar fusion, but that is stellar alchemy nonetheless).

The point is that in the `old days', most of the universe was made of these atomic building blocks. The universe was just made of either empty space or some heap of atoms (which make the heap of clothes on your floor). Most of the atoms which constitute the heaps of stuff in the universe were built up from the lightest atoms through thermonuclear fusion inside of stars or during supernova explosions. And that explained all the galaxies and dust and stars and planets: the universe.

Atomic Masses

Another important point here is that the negative electrons balance the positive charge of the protons in the nucleus, but they are very much lighter than protons and neutrons. The proton has about the same mass as a neutron, but the electron's mass is about 1/1837 of the proton mass. Therefore, when considering the cosmological mass scales, we are concerned only with the contributions of nuclei of atoms, since their electrons are so light. In other words, if you wanted to know how much all of the atomic matter in the universe weighed, you could leave out the electrons and it would weigh about the same. That's why we call A=Z+N the atomic mass number, since the mass of each atom is by far just the weight of all of the protons and neutrons.

Nucleons, baryonic matter

Since protons and neutrons are generally found in the nuclei of atoms, we call them nucleons. A nucleon means either a proton or neutron. Another name for a proton or neutron is baryon. There are other baryons besides the proton and neutron -- it is a more general class of particle. This is explained later.

The important point is that cosmologists refer to ordinary atomic matter as "baryonic matter" since most of the mass of every atom or ion is in the nucleus and the nucleons (protons and neutrons) are technically classified as "baryons". So, when you hear or read about "baryonic matter" in the universe, it is really just ordinary atomic matter being discussed.