People used to say the same thing about the "luminiferous aether," you know.
Personally, I think "dark matter" and "dark energy" don't really exist. Instead, I think there's something wrong with our understanding of the fundamental forces of the universe. Perhaps gravity doesn't behave with the inverse-square law across vast distances like we think. Perhaps there's a subtle force out there we've yet to discover that only acts over extreme distances. After all, quantum mechanics is only observable at extremely small scales, and a century ago nobody even suspected it existed. What's to say there's not something else that acts in an observable fashion only at galactic scales?