geometry of our Universe smoothed on large scales. Besides, the use of the perturbation theory allows one to understand the properties of the large scale structure, as well as its growth from initial conditions set by inflation and constrained by the observation of the cosmic microwave background.

While this simple solution of the Einstein field equations, together with the perturbation theory, provides a description of the Universe in agreement with all existing data, it raises many questions on the reason why it actually gives such a good description. In particular, it involves a smoothing scale which is not included in the model itself [@r.scale]. This opened a lively debate on the fitting problem [@r.fitting] (i.e. what is the best-fit FL model to the lumpy Universe?) and on backreaction (i.e. the fact that local inhomogeneities may affect the cosmological dynamics). The amplitude of backreaction is still actively debated [@buchertetal; @nobackreac; @backsig], see Ref. [@review-backreac] for a critical review.

Regardless of backreaction, the cosmological model assumes that the distribution of matter is continuous (i.e. it assumes that the fluid approximation holds on the scales of interest) both at the background and perturbation levels. Indeed numerical simulations fill part of this gap by 