From what I’ve heard, there is no absolute grid you can put on the universe, and therefore you can only ever talk about relative motion.
I’ve also heard that energy is conserved and constant no matter your location or velocity.
But how does this work with kinetic energy? Surely if I start walking in a certain direction, all the galaxies in that direction have decelerated, and those behind me will have accelerated by the same amount. But since the equation for kinetic energy is $E = (1/2)mv^2$, this means that the galaxies which have, from my perspective, accelerated, have gained more kinetic energy than those which have, from my perspective, decelerated. How does this work? I’m assuming that the energy changes that happen when I walk are insignificant compared to the extra kinetic energy gained by billions of galaxies.