This thesis discusses modern topics in particle phenomenology. In particular, there is an emphasis on physics coupling to standard model particles in an extremely weak way. We review the motivations for the inclusion of such physics. This kind of physics can take the form of additional new particles like axions or dark photons, which are themselves enticing dark matter candidates, or alternatively the force of gravity, which is known to couple very weakly to all the matter we see. In the former case a multitude of experimental strategies are required to probe the dark matter candidates' parameter spaces. Some ways to detect these new particles are discussed in the thesis, ranging from dedicated table-top scale laboratory searches to using natural features of the Earth's ionosphere for detection. The prospects of these different methods are expanded upon. In particular, we look at how to utilise spin-precession experiments for couplings to light, a prospective radio-antenna experiment to search for a signal originating from resonant conversion in the ionospheric plasma and also experimental bounds for the quadratic coupling of axions to light. Then there is a dedicated discussion for the self-consistency of a given effective quantum field theory (EFT). One such requirement is that time-like evolution is unitary. In conjunction with this an EFT is assumed to be local and causal. Combining these requirements leads to `positivity' bounds for the coefficients in the EFT. We study these bounds in the context of very low energy theories involving only massless particles. We address problems that occur from taking the forward-limit when finding these bounds. This results in conservative bounds on the low-energy coefficients.