A not so widely known fact about 'Space'

Any volume of space containing no particle of matter, is still not empty. Even the absolute vacuum has a pressure measure of zero but in universe there are places which have negative pressure.

This proves that even when there is no matter available, we can not call it empty and thus space is not empty but filled with something which is the origin (zero) of the frame of reference for all the matter (not for dark matter). This is why we think that space is empty while the reality is that, it is not.

Medium of Light

There is no luminiferous aether but that doesn't mean that space is empty. It is made up something which provides medium to light. Einstein says any such things would slow down the light. Yes it is slowing down the light that is why light is limited to 3,00,000 Km/s.

Speed of a wave depends on the properties of its medium. Space is determining the speed of light and thus the scale of time that is why there is a sequence of events in the universe. In the absence of this limit to the speed of light, all events would have happened instantaneously.

Relativity of Space

Perfect vacuum is just the origin of our reference frame of space. It is not the absolute zero. It means that space can be negative too. At least to some extent if not to the negative infinitive.