Before the time when the universe first became neutral (recombined) at z approximately 1100, the ion density was very high so that the mean free path of light photons was small enough that you could consider the universe as opaque. This opacity is what we see when we look at the Cosmic Microwave Background (CMB). After the recombination, the universe was transparent to visible light, but 'rapidly' (a million years or so) became dark as the CMB radiation was redshifted into the infrared. It stayed dark (in visible light) until the first stars formed. These stars also re-ionized the universe, a process that was completed by redshifts of 10-20, a few hundred million years after the Big Bang. But this time, the universe stayed essentially transparent because with the scale factor increasing by a factor of about 100, the density of the universe had decreased by a factor of a million, and the opacity varies as the square of the density.
So, the universe, with the exception of dense dust clouds, has been transparent to visible light ever since the recombination at the time of the CMB. But there is an important issue when we try to look back to that time in visible light or the near infrared (as with Hubble). That is, the neutral hydrogen which dominated from z = 1100 to z = 10-20 is strongly opaque to far ultraviolet light. First there are a series of absorption lines, called the "Lyman Series" (see wikipedia), starting at wavelengths 121.6 nm (nanometers) where the sole electron in a hydrogen atom is kicked from the ground state into various excited states, followed by the Lyman continuum, starting at 91.2 nm whereby the electron is kicked out completely, temporarily ionizing the atom. So, a universe of neutral hydrogen is opaque at wavelengths less than 91.2 nm. But look back to redshifts of 9, and that limit becomes 912 nm, well into the infrared. So Hubble becomes stymied at redshifts much greater than this.
Indeed, the Lyman series of absorption lines starting at 121.6 nm has long been, using ground-based telescopes, from remnants of neutral hydrogen clouds along the lines of sight from very distant quasars, and dubbed the 'Lyman forest' in these spectra.
So there is a second meaning to 'dark ages' when considering our present attempts to look back into the neutral era. The James Webb space telescope is designed with far infrared detectors to probe back into the 'dark ages' to see what was emitted from the earliest stars as visible light.