In the theory of relativity, spacetime is indeed considered as a concretely existing entity, but it is not associated with an absolute frame of reference or a preferred state of rest. The fundamental principle of relativity, as proposed by Albert Einstein, is that the laws of physics are the same in all inertial frames of reference. This means that no particular frame of reference can be considered as objectively "at rest" or in a privileged position.
The absence of an absolute rest frame implies that the concept of motion itself becomes relative. An object's motion is always described relative to another object or frame of reference. Different observers, moving relative to each other, may have different measurements of time, length, and the order of events. This leads to the phenomenon of time dilation, which is symmetrical in the context of special relativity.
In special relativity, time dilation occurs due to relative motion between observers. If two observers are in relative motion, each will measure the other's clock to be running slower compared to their own. This symmetry arises because the laws of physics, including the constancy of the speed of light, hold true for both observers.
In the case of gravitational time dilation, which is a consequence of general relativity, the situation is more complex. Gravitational time dilation is related to the curvature of spacetime caused by massive objects. This curvature affects the passage of time, and as a result, clocks in different gravitational fields can tick at different rates. However, it's important to note that gravitational time dilation is still a relative effect and depends on the local gravitational field strength. It is not related to an absolute rest frame.
The absence of an absolute rest frame and the symmetrical nature of time dilation are fundamental principles of relativity theory, which have been extensively tested and confirmed by experimental observations.