What differentiates the theory of dark matter from the theory of ether? What makes it something more than just a redefinition of the concept of ether?
Er, dark matter has absolutely nothing to do with ether.
Ether was the supposed medium through which light waves propagate. IIRC, it was not thought to have any mass.
Dark matter is matter that does not emit radiation, but which is detectable through its gravitational influence.
Perhaps you could be clearer about why you think they could be related?