Surely it would be better to say that "we currently have no conclusive evidence that HGRV's exist", rather than saying that they don't exist? That seems like a reasonable approach to me. To say something doesn't exist in science, just because you don't have complete evidence for it, seems wrong to me. In physics, they are always looking for yet-to-be-discovered but predicted sub-atomic particles, such as the 'Higgs Boson' which they currently have no physical evidence for, but only theories or hypotheses that the particle fits into. They don't say that the Higgs Boson doesn't exist at the moment, because that would preempt the search for it, and pre-judge the outcome of the research... They say that it could potentially exist but that they don't have physical evidence for it; and that they are currently looking for physical evidence for it. If a scientific consensus develops for some other theoretical model of physics, that the Higgs Boson doesn't fit into, based on findings in the actual physical research, then there might be a consensus that the Higgs Boson doesn't exist, in which case they would say that the Higgs Boson doesn't exist.