If the Peccei-Quinn symmetry associated to an axion-like particle has ever been restored after inflation, axion strings and domain walls inevitably form. I will argue that these topological defects can have interesting experimental and observational consequences. In particular, If the axion decay constant is larger than 10^14 GeV the strings produce an observable stochastic gravitational wave background. In other theories, the contribution to the relic abundance from domain walls allows an axion with a relatively small decay constant, and therefore a possibly large coupling to photons, to comprise the full dark matter abundance. The relic axions are produced with a spatial distribution that might lead to dark matter substructure, which, in some cases, forms in a regime such that quantum pressure is relevant.
I present a new probe of purely gravitationally coupled sectors with large anisotropies. These anisotropies are damped via gravitational interactions with the baryon-photon fluid, which is heated up in the process. The injected heat causes measurable distortions of the cosmic microwave background spectrum. This method can be applied to anisotropies in the form of a domain wall/cosmic string network or caused by a first order phase transition or scalar field dynamics. I show that this method can potentially probe large regions of previously unconstrained parameter space and is very much complementary to up-coming searches of gravitational waves caused by such dark sectors.