  We use that gravity probes only the total energy momentum tensor to show how
this leads to a degeneracy for generalised dark energy models. Because of this
degeneracy, Omega_m cannot be measured. We demonstrate this explicitely by
showing that the CMB and supernova data is compatible with very large and very
small values of Omega_m for a specific family of dark energy models. We also
show that for the same reason interacting dark energy is always equivalent to a
family of non-interacting models. We argue that it is better to face this
degeneracy and to parametrise the actual observables.
