The difference is that there is evidence fir the existence of dark matter, namely, the rotational behavior of galaxies that can’t be explained by their observed mass, whereas there is no evidence for the existence of ether, and in fact there is evidence it does not exist, namely the outcome of the “aberration of light” experiments, which gave null results.
... and the Michelson-Morley experiment, which was recently repeated with more decimal places of zero.