a simple example that we can comprehend: if you are in a train with closed windows, it is impossible to know whether the train is stopped or moving with perfect constant velocity, with no acceleration. So, it makes no sense to ask “am I truly at rest”? being at rest is same as moving at constant velocity.
the reason we think there is a “center”, because the center is “stationary” while the rest are moving with acceleration around it.
BUT similarly to the above (from memory, not gonna to revisit Einsteinian physics until the kids get there 🙂 turns out when there is acceleration, you will not be able to distinguish it from gravity. At the end, you can write down equations using any accelerating point as a “center” (0,0,0) and describe all other movements relative to that.