Ether was invoked the structure of space was necessary for fields to propagate, e.g. light was a vibration of space. This is no longer necessary. And could not be detected in the M-M experiment.

The Ionian Greeks proposed most of same models of space as modern physics, but lacked experiments to select the correct model. Some model propes emptiness and others an underlying substance of matter or energy.

It could be infinity. We don’t really don’t know. But it could be. It has to be something, but it could be infinity, right?

- DJT

At the macro level, Space could expand outward infinitely, or maybe if you travel further enough and you return to where you started. Or maybe there's the multiverse thats teaming with infinite universes?

Most physicists will tell you that at the quantum level, the most fundamental level is at the Planck scale, and the "nothingness" of space is still teaming with particles popping in and out of existence. Some particles being hypothetical and have yet to be discovered.