Aether seemed to be necessary conceptually because the theory of electromagnetism was a theory of waves, and no would could conceive of waves except in some medium, hence aether. Einstein's breakthrough was to show that the concept of aether was not necessary to explain empirical observations about electromagnetism and the speed of light.

Dark matter, on the other hand, is not a conceptual necessity but an empirical one. Whereas aether was something people clung to because they couldn't imagine a world without it, dark matter is something scientists have been forced into even though the world would be more conceptually elegant without it.