Baryons are types of subatomic particles. They include protons and neutrons as well as other kinds that only exist briefly in an atom smasher. In this case, the

Baryons are types of subatomic particles. They include protons and neutrons as well as other kinds that only exist briefly in an atom smasher. In this case, they are using a generic term because they have no way of knowing whether it's primarily protons/neutrons or what else it could be made of. A gas in this sense is really a very loose collection of light particles, mainly hydrogen/helium or just empty separate protons and neutrons. As opposed to heavier atoms that are the product of stellar fusion and form more solid matter. If you think about this in terms of cosmology, the universe was originally a solid uniform blob of high energy particles. As it cooled, some of those particles grouped together to form stars and galaxies. However, there are parts in between galaxies that clumped together a bit but didn't really have enough mass to form anything. So it's really just made of tendrils of basic atomic building blocks.