Imagine pure space without a single object in it. Not even you, the observer, but nonetheless for some reason you are still able to “observe” this space.
In what way can you differentiate one dimensionless point in this space to another dimensionless point? By what means can you understand the distance these two dimensionless points? How do you discover the size of this space? Or else does this space have a size at all? And how do you differentiate between a point and space, when there is no way to differentiate between the two?
Democritus and Leucippus called this void, but only when compared or contrasted to atoms. In other words, void is defined by the things, or the absence thereof, not by any positive means.
Parmenides found this troubling. For then void cannot be truly void. Nothing must be “something”. He drew the conclusion that all is one, and that change as observed in the world is an illusion.
Einstein described with the equation E=mc2 the world as mass, space, time, and energy. For any one of these elements to have a value would mean the entirety is zero, nothing or void. But nothing seems to have zero energy. Atoms brought to near zero Kelvin slows motion but can never stop it. Everything is in flux as Heraclitus had concluded. So space is energy, and not empty void as such.
Space is likely a thing, but because of its nature as a “homogenous” thing it cannot be observed directly but only by indirect means or inferred through the relationship of things.