Thanks... I think! I will read the various linked references and see if that helps. Of course the fact that I don't know anything of relativity (beyond the usual basic grasp) or of advanced mathematics means I am up against it!
Some of the replies talk of "space-time", again in terms that suggest it is a thing. So I think a part of my conceptual difficulty is grasping what this means in a physical sense. I thought space-time referred to how the trajectories of phycial entities is affected over time by gravity (that is, for example, curved space describes how objects trajectories "curve" due to gravitational forces), rather than referring to anything through which objects move. The Einsteinian Field Effect equations referred to earlier seems to my naive eye to represent that physical fact by way of a mathematical abstraction that can be used to predict this effect for different physical scenarios (naive eye, I said - don't let the fact that I used the words Einsteinian Field Effect equation suggest I have any idea what I am talking about!).
If that were the case, then wouldn't we be describing not a place or a location but a relative motion that depends entirely for its force upon physical entities? And if space is not a physical entity, which to my mind it isn't, how can the equation, or the terms in the equation, describe anything about it?
I suppose I am not being clear here. I just am struggling to see what space, or space-time, actually IS. Or how its expansion has any effect on anything, again because to me, that can't happen if it doesn't have physical form (space is distance, not a physical and hence describable thing). I can totally understand that space and space-time, defined mathematically, can describe the behaviour of physical entities, but that's not space or space-time per se. When I think about this, it seems to me that objects, say galaxies, travel "through" fields (say gravity), but not through space. Their motions only exist relative to each other, which we can only describe in terms of some coordinate system that references the entities concerned. Are we really talking about a thing or rather relationships?
Sigh... what am I missing or is this just too hard for me?