Watched several shows on gravity, and relativity, and still my question goes unanswered for me. What IS space? It has to BE something if it can bend or expand!!! Searching . . . searching . . .My reply...
Trying to find an absolute definition of spacetime beyond just a physical manifestation of a mathematical function isn't going to get you very far. It's something that exists in this universe not as a tangible object, but as a structure of the universe itself, riding the line between what is in our universe and what is not. It serves as a mathematical foundation for the other laws of physics to structure themselves on. There is no intuitive way to understand it because our brains have evolved to process only tangible concepts. In it's essence, every facet of the universe can almost certainly be broken down entirely into mathematical functions and probability ratios.
As I understand it, spacetime is the bounds of our three dimensional universe wrapped around itself into an expanding four dimensional hypersphere. Spacetime is the surface of our three dimensional universe where it contacts the fourth dimension. Being that spacetime is absolutely everywhere, that means that in every space down to the infintessimally small is almost infinitely close to touching the fourth dimension, but never will. Whether anything does exist or even whether it is possible for anything to exist within that fourth dimension is likely impossible to ever know, as it is beyond the reach of our universe. (by fourth dimension, I mean a fourth spacial dimension, not time). This also means that you can't point to the direction where the big bang happened, because the big bang happened everywhere, and in a sense is continuing to happen everywhere even as you're reading this, in the form of universal expansion.
When dealing with things in the realm of astrophysics, it does no good to try to rationalize things as anything other than mathematical functions, because essentially, that's what everything really is. We simply perceive the very obtuse surface effects of an infinitude of mathematics continuously unfurling at blistering speed.
Of course you already know, but it's worth mentioning that what you experience isn't reality, only a conglomeration of information gathered by your organs and processed by your brain. As is evident by the existence of superstitions and religions, our brains are not so good at putting all of this information together beyond what is necessary to keep our biochemical machinery functioning long enough to replicate itself.
Spacetime bothers me too when I try to conceptualize it. (just like everyone else on earth I'm sure). Still, it doesn't frustrate me as much as trying to understand quantum mechanics and Heisenburg's uncertainty principle. Everyone (kind of) knows that light acts both as a wave and a particle. That's taught in grade school. What they don't teach in grade school is that it behaves that way because it does not physically exist until it is acted upon. It exists only as a wave of probability. A mathematical function.
Here's a good vid on one facet of quantum mechanics if you haven't already seen it...
There's a great article I read involving lasers and prisms demonstrating how you can RETROACTIVELY change the path a photon takes (took) as it is (was) in motion. I can't seem to find it now, but it was a fantastic article.
Or, you could just say "goddidit". It's a lot easier and seems to be the popular way to go.