Concider a patch of the universe where there is only negligible gravitational curvature. In this patch, SR applies. We know therefore that there is no preferred intertial motion there. But with background radiation taken into the picture, that seems not to be the case anymore. I've read the phrase "being at rest wrt the Universe's expansion", which I take to mean a state where the background radiation is identical in intensity everywhere (roughly). If we are not in such a state, there is a direction from which the intensity is greater then other directions, and this should act as a sort of "intrinsic friction" of the universe. Of course, today such radiation is negligible, but earlier in the Universe's lifetime it was considerable.