[nyc111]

"Spacetime is not a physical thing."

But physicists shamlessly reify spacetime. How can something that is said to have a "fabric" not physical? How can something that expands not be physical.

At this point physicists enter into semantics and ask, "it all depends on how you define 'physical'". And indeed, physicists heavily use casuistry, and they have a definition for every case.

To me, if spacetime is not physical, then, it does not exist. Similarly, I don't think such mathematical abtractions as "point particles" exist. Physicists do not worry about existence. They only care to get a prediction.

[r0uv3n]

Spacetime has a notion of distance between its different points attached to it. These distances are important for physics. When people say that spacetime "stretches" or "expand", they mean that these distances change.