Well, the problem is there could still be another resolution of the problem, based on something else we haven't thought of yet. Dark energy is still ad hoc ; it's not directly observed, but rather hypothesized to make the equations fit. The GR equations reconcile a lot of observations, and so shouldn't be cavalierly discarded; but at the same time, 'dark energy' doesn't have the same level of certainty, as, say, neutrons.
I think the neutrino analogy is valid. Fermi came up with neutrinos because conservation laws were apparently being violated, and nobody wanted to throw out conservation laws simply because of anomalies in one phenomenon. However, IMO, neutrinos were still a rather dodgy ad hoc hypothesis until they (or their effects) were observed directly.
We should have a special category of 'stuff', that exists because we need it to exist to keep a valuable theory from falling apart, but where there's no other independent verification of its existence. And we should be careful not to talk about such 'stuff' in the same way we talk about more substantial stuff.
Having said that, I don't attend cosmology conferences, and I'm sure you're right that the guys in the field are probably appropiately careful about how they talk about such ideas; maybe the advice needs to be given to science journalists and not to cosmologists.
I think Physicist has indicated there are several lines of evidence (but I will allow him to explain what they are).
I would suggest that from a layman's viewpoint, a conceptual placeholder becomes "stuff" when its effects can be clearly delineated by independent avenues of research. Seeing and touching, metaphorically speaking.
It was actually Pauli who came up with neutrinos in order to retain the conservation laws, but it was Fermi who named them, and who formally described their behavior when he formulated his theory of nuclear decay.