matter content of our Universe that will allow us to interpret observations at all. In the following we assume only that:

1.  The geometry of the Universe is well described by small linear perturbations living in an FLRW metric with scale factor $a(t)$. We will not consider possible observations of rotational perturbation modes nor of gravitational waves, as these are irrelevant for structure formation in late-time cosmology.

2.  The matter content (i.e. dark matter and baryonic matter) is pressureless or evolves in a known way.

3.  The relation between the galaxy distribution and the matter distribution at linear scales can be modeled as $\delta_{\text{gal}}=b(k,a)\delta_{\text{m}},\,$ where $b(k,a)$ is the potentially scale- and time-dependent linear bias, while at the same time there is no bias between the velocities of galaxies and matter. This implies that both the baryonic and dark matter respond in the same way to the gravitational potentials and that the statistical velocity bias due to galaxies sampling preferentially over-dense regions [@Desjacques:2009kt] is negligible on the scales of interest.

4.  The late-time universe is effectively described by the action $$S=\int\text{d}^{4}x\sqrt{-g}\left(\tfrac{1}{2}R+\mathcal{L}_{x}+\mathcal{L}_{\text{m}}\right)\,,\label{eq:Action}$$ (setting $8\pi G_{\text{N}}=1$) which includes the Einstein-Hilbert term for the metric $g_{\mu\nu}$ and the Lagrangian $\mathcal{\mathcal{L}_{\text{m}}}$ describing pressureless matter fluids, 