Consider two inertial observers starting from the same spacetime point, one remaining at rest relative to the CMB and one moving at speed $c/2$.
According to relativity theory, neither observer has a preferred status - no local experiment would allow you to determine your velocity in an absolute sense.
However the first observer has a preferred status in that he observes the universe as approximately homogeneous and isotropic whilst the second does not.
This situation is often described by saying general relativity is a background independent theory but the background is determined 'dynamically'.
What is this dynamic mechanism which resulted in the first observer having this preferred status? I am just trying to get a handle on whether this is a meaningful question to ask within the current state of mainsteam physics or not. I will accept a simple "No" as an answer, but any justification would be appreciated.
Edit: I am not asking if the existence of a preferred frame contradicts relativity, but rather whether this situation points to a deeper more natural theory of spacetime where we don't have to just say "this is the universe we find ourselves in".
Edit: The two given answers has allowed me to give a more precise statement of this question: "Of the 10 Poincare group degrees of freedom, our universe appears not to have a preferred spatial origin (3, homogeneity) , not to have a preferred spatial direction (3, isotropy) but it does have a preferred temporal origin (1) and a preferred (local) Lorentz boost (3). Can we explain the latter of these?"