  Cosmological background observations cannot fix the dark energy equation of
state, which is related to a degeneracy in the definition of the dark sector
components. Here we show that this degeneracy can be broken at perturbation
level by imposing two observational properties on dark matter. First, dark
matter is defined as the clustering component we observe in large scale
structures. This definition is meaningful only if dark energy is unperturbed,
which is achieved if we additionally assume, as a second condition, that dark
matter is cold, i.e. non-relativistic. As a consequence, dark energy models
with equation-of-state parameter $-1 \le\omega< 0$ are reduced to two
observationally distinguishable classes with $\omega = -1$, equally competitive
when tested against observations. The first comprises the $\Lambda$CDM model
with constant dark energy density. The second consists of interacting models
with an energy flux from dark energy to dark matter.
