| Summary: | The fundamental nature of dark matter remains a mystery. As crucial evidence for its existence comes from its effects on early universe observables, perhaps clues to its fundamental nature also reside in its behavior in the early universe. In this thesis, I explore two scenarios in which dark matter is capable of exotic behavior in the early universe. In the first scenario, I consider particle dark matter that is able to decay or annihilate into standard model matter. I describe a code package, DarkHistory, that quickly and accurately calculates the effects such annihilations and decays have on the evolution of the ionization levels, matter temperature, and spectrum of photons in the early universe. I then use DarkHistory and measurements of the Ly𝛼 forest to place constraints on the decay lifetime and annihilation rates of dark matter. In the second scenario, I consider dark matter that consists of dark quarks and gluons. In the specific model I consider, the confinement phase transition is of first order, leading to the formation of bubbles. The dynamics of these bubbles and their interactions with the dark quarks dramatically modifies their present-day abundance.
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