Cosmic Reionization (z ≈ 6–12): After recombination, neutral hydrogen filled the Universe — the "cosmic dark ages." The first stars (Pop III, M~100 M☉) and quasars emitted UV photons that carved ionized bubbles (Strömgren spheres) into the neutral IGM. These bubbles grew and eventually overlapped, completing reionization by z~6. The Gunn-Peterson trough in quasar spectra traces this transition. CMB polarization (Planck 2016) gives τ_reion ≈ 0.054.