[IanCal]

I think really this is leading into much more philosophical discussions. The practical viewpoint is I think 'anything that cannot interact with me can safely be ignored as irrelevant'.

But to push into a few thought experiments:

If I go into a room and it is sealed up so that nothing at all may escape and the room may never be opened, do I cease to exist? Or do you? Do we now have two realities? Is it possible to construct a setup where discrete realities no longer explain the true nature? Can I construct a system where two agents will disagree on where the boundaries are between these realities? Since we can create things where two parties cannot agree on the ordering of events, perhaps this could be extended to identify a setup and point in time where two parties would disagree on whether or not something exists (I'm really not sure on this, I feel the answer is no but then I think that about other things that smarter people than me can construct).

Do you consider only current technology or must we also consider things we don't yet know? When humanity was in its early stages, did one tribe not exist to the other? Did some countries not exist?

Do things "exist" if they don't currently interact, and may never interact, but possibly could?

The practical conclusion is perhaps that the term "exist" isn't very helpful.