Hi azawah,
[sorry, This post got way too long...but with luck I'll have explained, oops! wrong word...discussed some things of interest..]
Two general points:
The single most importanent thing gravity does is to draw mass and energy together...to form stars, planets and eventually,...us. If it did not we would not be here; sure they are many other essentials, but those are key.
Second, no one knows what space is, let alone curved spacetime [gravity], any more than they know what time is or mass, energy, gravity,etc,etc. For most part, we can describe the observational effects of all these but not the fundamental origin nor "what is...made of?" what we usually discuss is 'How does THAT behave...' One thing we do think we know : 'space' or, 'spacetime' if you prefer, is NOT 'empty'...
Here are brief quotes from another discussion on particles [think, 'mass' if you like] to illustrate some of the subtlies underlying gravity and particles:
One comment was made:
As a general rule the world is not made of particles, it is more correct and less confusing to say that it is made of fields. Unless I'm mistaken all or most of us at the Forum realize this?
[This post refers to 'fields' which have never been directly observed; they are aspects of mathematical models and are pervasive in phsics..because they work. Are fields real or just imaginary descriptions, like lines of lat and long we place on a map??
Referring to the above quote:
Marcus:
I don't count myself in this group. As Naty1's quote said "Particles are the objects revealed by detectors, tracks in bubble chambers, or discharges of a photomultiplier." This means that particles (not some mysterious fields) are the objects studied by real experimental physics. If "curved spacetime" does not agree with the particle concept, so bad for the "curved spacetime".
[So here we were discussing the ambiguity of gravitational spacetime curvature [gravity] with respect to particles. My quote above is actually from a Carlo Rovelli research paper we were likely discussing.]
What this gets at is that local particle states correspond to the 'real' objects observed by finite size detectors. On the other hand, global particle states can be defined only under certain limited conditions. With gravity, global distances and times [in curved spacetimes] are indeterminate as well...they are not typically subject to precise definition. And, this is really crazy, we cannot ascribe precise energies to a local gravitational field. So one smart poster here says:
Matter is that which has localized mass-energy, while spacetime does not.
Take 'matter' here as 'particle' if you like.
Want to know three ways to 'create real particles' from 'nothing'?? that is, be able to measure something locally via an appropriate detector that was 'not there' before: accelerate, accelerate spacetime [via expanding horizons as in our universe], accelerate a cosmological horizon.
[If this seems interesting try reading 'Unruh effect' in Wikipedia...you will be amazed! I know I still am.
What to make of all this?? It goes back to the 'complicated' answer of Dr Chinese above. We have a lot more to learn before we have a 'comprehensive theory'! How do all the pieces fit together??
