Theoretically, can perfectly flat spacetime exist in the Universe?

[Ibix]

Perfect flatness would require absolutely nothing in the entire universe

This is not correct. As already discussed in this thread, a spherical shell of matter in an otherwise empty universe has flat spacetime in the cavity inside.

You still can't have perfectly flat spacetime in the real universe, but your claim misses a case.

 

[PeterDonis]

This thread is now closed since the OP question has been well addressed.