Modified Gravity and Dark Energy can introduce changes in the gravitational potentials \(\Phi\) and \(\Psi\) and therefore influence the background expansion and the evolution of the perturbations of matter in the Universe.
 One can divide theories beyond the standard model in two classes, those with universal coupling and those with non-universal coupling. Universal coupling means that all particles feel the same forces, follow the same geodesics and interact in the same way with the extra field. In non-universally coupled theories, there is usually only one particle species that feels the dark energy field, for example Dark Matter or neutrinos. This approach is generally used to avoid the tight solar system constraints on modifications of gravity that apply to normal (baryonic) matter.
In my work I have dealt with very general theories of universally-coupled modified gravity, such as Horndeski and Effective Field Theory of Dark Energy (EFToDE), as well as with more specific theories such as coupled Dark Energy (cDE), in which there is a direct exchange of energy and momentum between DM and DE, or Growing Neutrino Quintessence in which neutrinos and the Dark Energy scalar field are strongly coupled to each other. I will explain more about these theories later on.