Presumably we should be able to build a theory of spacetime that yields no need for dark matter, but we're not there yet. On the other hand, determining the lo

Presumably we should be able to build a theory of spacetime that yields no need for dark matter, but we're not there yet. On the other hand, determining the local time dilation factor based on all mass beyond the local area is essentially not possible. We can talk about how the great voids have less time dilation than galaxy clusters, sure. But what about the universe as a whole? Our universe could be embedded in a larger one that contributes to time dilation in ours and we could never sense that. Time dilation at cosmological scales is relative for this reason.