  In this paper, we propose to use the mimetic Horndeski model as a model for
the dark universe. Both cold dark matter (CDM) and dark energy (DE) phenomena
are described by a single component, the mimetic field. In linear theory, we
show that this component effectively behaves like a perfect fluid with zero
sound speed and clusters on all scales. For the simpler mimetic cubic Horndeski
model, if the background expansion history is chosen to be identical to a
perfect fluid DE (PFDE) then the mimetic model predicts the same power spectrum
of the Newtonian potential as the PFDE model with zero sound speed. In
particular, if the background is chosen to be the same as that of LCDM, then
also in this case the power spectrum of the Newtonian potential in the mimetic
model becomes indistinguishable from the power spectrum in LCDM on linear
scales. A different conclusion may be found in the case of non-adiabatic
perturbations. We also discuss the distinguishability, using power spectrum
measurements from LCDM N-body simulations as a proxy for future observations,
between these mimetic models and other popular models of DE. For instance, we
find that if the background has an equation of state equal to -0.95 then we
will be able to distinguish the mimetic model from the PFDE model with unity
sound speed. On the other hand, it will be hard to do this distinction with
respect to the LCDM model.
