Gas

Once recombination had trapped all the electrons inside atoms, the fog of the Universe cleared. The young Universe was full of hot atoms, moving around and bouncing off each other. They made a gas. A gas is a collection of atoms (and perhaps molecules) moving about in space and sometimes hitting each other. The gas contains heat because of the movement energy of the atoms and molecules it contains -- the faster they move the hotter the gas.

The young Universe was filled with a hot gas of hydrogen and helium atoms.