luminiferous aether
The luminiferous aether was a hypothetical, invisible substance that nineteenth-century physicists believed filled all of space so that light would have something to wave in. Sound needs air, and ripples need water, so it seemed natural that light waves needed a medium too. The aether was imagined as the still, universal stage against which all motion could finally be measured as truly absolute.
To do its job the aether had to be a paradox of a material: utterly rigid enough to carry light's enormous speed, yet so thin and frictionless that planets sweep through it for billions of years without slowing. Physicists tied themselves in knots trying to give it consistent properties. If it existed, Earth's motion through it should produce a detectable 'aether wind', subtly changing the measured speed of light depending on direction.
Every attempt to catch that wind failed, most famously the Michelson–Morley experiment, which found no difference whatsoever. Einstein's 1905 special relativity then made the aether unnecessary: light needs no medium, and there is no absolute rest frame to measure against. The aether is a textbook case of a reasonable idea quietly retired once experiment and a better theory left no room for it.
Earth's orbital speed should have shifted the measured speed of light — but no such shift was ever found.
Abandoning the aether did not mean empty space is featureless. Modern physics fills the vacuum with quantum fields, but these provide no preferred rest frame, so they do not revive the old aether.