Interstellar gas clouds contain about 50% of the mass of the gaseous interstellar medium within galaxies, but occupy only 2% of the volume of interstellar space. The remaining mass is distributed throughout the rest of the galaxy and is known as intercloud gas, the bulk of which has temperatures of around 8,000 Kelvin and densities ranging between 0.01 - 1 atoms/cm3. This is known as the warm intercloud medium.
The hydrogen making up the warm intercloud medium is present in both neutral and ionised forms. The high-energy UV photons responsible for the ionisation of hydrogen atoms in HII regions are not only found close to hot, massive stars. Once they leave the star, they are free to travel through interstellar space until absorbed by an atom further down the line. Roughly half of the hydrogen atoms in the warm intercloud medium are ionised by these photons and can be observed as low surface brightness Hα emission. Approximately 90% of the ionised hydrogen in our Galaxy is contained within the warm intercloud medium with only 10% shining brightly as impressive HII regions.
In regions where there is a lot of warm intercloud gas, the ionising photons will eventually be used up, allowing the hydrogen to remain in its neutral state. These areas of neutral hydrogen are generally protected from UV photons by surrounding regions of ionised warm intercloud medium and are traced by the 21cm spin-flip transition, similar to what is used to locate HI clouds.