# What was the significance of the ionization caused by the Population III stars?

I am reading that the ultraviolet light that radiated from the first stars would ionize the surrounding gas and apparently, all of the matter in the universe would eventually become ionized.

So, from what I understand, instead of a gas, everything was plasma at this point.

What was the significance of this? did the plasma eventually turn back into a gas structure?

Pop III stars and dwarf galaxies at $Z \sim 6-10$ are not observable - but reionisation is! So it offers a probe or diagnostic of our understanding of these events. For instance, neutral gas strongly absorbs at the wavelength of Lyman alpha. Sight lines to very high redshift quasars show a forest of redshifted Lyman alpha lines from clouds of neutral gas spread out in the universe in their direction. However at redshifts beyond reionisation we expect most of the gas to be neutral right back to the redshifts of the CMB. This should show up as a broad "Gunn- Peterson" trough, and has been identified at $Z>6$. It is also thought that deep 21cm radio surveys can probe reionisation.