The Many Lives of Dark Matter: WIMPs, FIMPs, and EFTs

From Weakly Interacting Massive Particle (WIMPs) to Feebly Interacting Massive Particles (FIMPs), the landscape of Dark Matter models has expanded as experimental constraints have evolved. This thesis follows that evolution. We begin with the standard WIMP paradigm, where increasingly precise experiments have imposed stringent bounds on the simplest realizations. A number of well-motivated extensions, such as models with additional scalar or gauge bosons, are reviewed to illustrate how Dark Matter searches have evolved alongside these constraints. To address the tightening bounds, we next examine scenarios in which Dark Matter–nucleon scattering arises at loop level. Such loop-suppressed interactions alleviate the tension with direct detection experiments while preserving the thermal relic picture, thus keeping WIMPs viable within a modified framework. We then turn to the freeze-in mechanism, where Dark Matter is produced through feeble couplings in the early Universe. While these scenarios naturally evade direct searches, their tiny couplings make them almost impossible to test. To open up new possibilities, we explore a non-standard cosmological history with a low reheating temperature. In this setup, viable parameter space emerges with couplings large enough to be experimentally probed, providing a striking contrast with the WIMP case. Finally, we move beyond specific models to an Effective Field Theory (EFT) approach, where mediators and interactions are organized in a systematic, gauge-invariant framework. This extended EFT captures the essential features of a broad class of ultraviolet completions and provides a model-independent language to confront theory with experiment, offering a versatile tool for future Dark Matter searches.