When gas expands to fill a container it's because it's under pressure and/or excited. It's not something that magically happens all the time no matter what.
In order for it to be a gas, that already means it is excited. Otherwise it would be a less excited state, a solid or a liquid. Expanding to fill a container is a property of all gasses, yes, they do always fill the container. It is a basic property of a gas.
Compared to the other states of matter, gases have low density and viscosity. Pressure and temperature influence the particles within a certain volume. This variation in particle separation and speed is referred to as compressibility. This particle separation and size influences optical properties of gases as can be found in the following list of refractive indices. Finally, gas particles spread apart or diffuse in order to homogeneously distribute themselves throughout any container.
In order for it to be a gas, that already means it is excited. Otherwise it would be a less excited state, a solid or a liquid. Expanding to fill a container is a property of all gasses, yes, they do always fill the container. It is a basic property of a gas.
Under Physical Characteristics
hi. its comfy way down in here.