There are many ways to search for new particles beyond the Standard Model, such as detecting a relic dark matter abundance, or production in high-energy processes. To understand these, one needs to calculate how the new particles interact with Standard Model matter. Sometimes, one can simply calculate Feynman diagrams between a few particles. However, in dense Standard Model environments, coherent interactions with many Standard Model particles can be very important, and can affect the rates of processes involving new particles by orders of magnitude. In this seminar, I will discuss these effects in different scenarios, including particle production in stars, dark matter scattering in astrophysical systems and in the laboratory, and absorption of light bosonic dark matter in experiments.