I've been trying to learn about cosmological expansion (some weeks ago), I think I understand as much as any lay-person could, regarding why everything is moving away from our galaxy. However I still don't understand what spacetime is. The fact that space can deform indicates to me that spacetime is not not a metaphysical thing, but something that has tangible, observable properties. Such as in observing closer galaxies lens more distant galaxies, or that the Alcubierre drive is theoretically possible...or moreover that a 'big rip' could tear the electrons from atomic nuclei, if expansion accelerates. So if the space part of spacetime itself is getting bigger, and for the aforementioned reasons space is presumably more than just a vacuum of quantum mechanical fluctuations (with various standard model fields in it). Then does humanity actually know what spacetime is, or is it still more or less a mystery? Thank you P.S. I forgot, of course gravitational waves.