Metal hydrides show spectacular phenomena as a function of the hydrogen concentration such as a drastic optical change caused by a metal-to-insulator transition and high-temperature superconductivity. Yet, little is known about their electronic properties. In order to better understand the physics of the parent compounds, we here study the optical properties of yttrium hydride YH_x for 0 ≤ x ≤ 3, where x directly relates to the charge carrier density, using broad-band optical spectroscopy. We record the transmission of 200 nm Y thin films, capped with 5 nm Pd as a catalyst, in the presence of 25 mbar H₂ gas between 0.25 and 650 THz as a function of time. From a spectral weight analysis, we find a dynamic and correlated behavior of electronic and vibrational spectral weights as a function of photon energy. The spectra are dominated by a Drude contribution which rapidly decreases across the metal-to-insulator transition. We reveal two regions of additional spectral weight, in the far-infrared for 0 ≤ x ≤ 2 + ε and in the mid-infrared for 2 ≤ x ≤ 3 - δ which are dome-shaped as a function of the charge carrier density. Both weights show a concomitant behavior with the Drude weight, suggesting the importance of correlations in the parent compound of yttrium hydride high-temperature superconductors.