  We analyze some generic properties of the dark energy (DE) perturbations, in
the case of a self-conserved DE fluid. We also apply a simple test (the
"F-test") to compare a model to the data on large scale structure (LSS) under
the assumption of negligible DE perturbations. We exemplify our discussions by
means of the LXCDM model, showing that it provides a viable solution to the
cosmological coincidence problem.
