The present-day universe formed out of an initially hot, dense, rapidly expanding plasma, which cooled during the expansion to become a neutral gas. Under the influence of gravity, small initial fluctuations in the density grew over time until the first sources of light formed within the neutral gas, starting to reionize the universe. These "dark ages of the universe" are explored in this book, and the current uncertainties concerning the physics governing this epoch are discussed. Particular focus is on the potential role of primordial magnetic fields, which may have been created during phase transitions in the early universe, as well as the effects of self-annihilating or decaying dark matter particles. Supermassive black holes, which are observed in some of the earliest galaxies, must have formed during this epoch and grown rapidly in mass. Possible probes of this early epoch are discussed, ranging from measurements of the cosmic microwave background with satellites like WMAP or Planck, future radio telescopes like LOFAR and SKA, measurements of the cosmic gamma-ray background, as well as observations in the mm- and sub-mm regime with the upcoming ALMA telescope.