While many of us are dazzled by the spectacular examples of galaxies each one with its own 10-100 billion stars, some of us choose instead to study the regions between the galaxies.
There is a fair amount of hydrogen gas in this “intergalactic medium,” yet this gas typically is too faint to see directly in images. We are able to study this dim gas only by looking at how it affects the light coming from bright objects in the background called quasars.
There are a great many bright quasars, or galaxies with extremely bright nuclei, in the universe. These quasars each produce a tremendous amount of light much like the welcome sight of a flashlight on an otherwise dark and deserted hiking trail.
Indeed if you were to look at the flashlight of a distant hiker during an evening walk, you may see the flashlight seem to ‘flicker’ not because the battery was running out but rather as a result of obstructions along the way such as tree branches. By conducting such an experiment you may be able to discern how dense the foliage is between yourself and the other hiker. Similarly, by studying the amount and placement of artificial ‘flickers’ in the quasar light caused by the intervening hydrogen gas, we can put together the properties of that elusive environment.
What we find is that hydrogen in the intergalactic medium near the Milky Way is completely ionized, which means that the electrons have been stripped from the protons by UV light. So, if you are wondering if a powerful sunblock should be in order should you ever find yourself in between galaxies that the answer is yes (although there would likely be many other, more pressing concerns like the lack of water)!
From such studies of the absorption of light towards quasars we also learn that at that at an earlier time in cosmic history, the intergalactic medium was neutral. During that time each proton was still attached to an electron.
An interesting question to ask is which event or events happened in the universe to cause the intergalactic medium to suffer a deluge of UV light which ionized all the hydrogen atoms? While the current cause of this “reionization” is still unknown, one hypothesis is that faint, small and fairly weak galaxies in the early universe were the culprits. Although the UV light production of any one 'dwarf' galaxy is small, they win owing to their steady production rate and large numbers.