This can be (kinda) explained via the notion of space-time expansion. (This is an intrinsic phenomenon, having nothing to do with objects moving through space.)

This can be (kinda) explained via the notion of space-time expansion. (This is an intrinsic phenomenon, having nothing to do with objects moving through space.) As the space-time expands, its density becomes lower. The (relative) change in density can be measured indirectly by measuring (the effects of) the expansion. A technical explanation of the expansion involves the inflaton field. (There's also energy density of the gravitational field.)