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Originally Posted by octothorp

Regarding dimensions:

Spoiler!

I think the question is why the upside down seems to mirror our world geographically; there are more hints (but still no answers) in the dimension than in the monster itself. Is this an alternate dimension that at some point diverged from ours? Is it a 'future' version of ours? Is it a different dimension that ours has somehow infected? Maybe even an artificially-created dimension? Also, what did the monster eat before it got access to our dimension, because that other dimension looks pretty barren for such a voracious carnivore.

Spoiler!

I always thought that the upside down was a dimension that always existed and was somehow the negative of ours. Everything in our dimension has a negative version in the upside down.

The monster was the result of the tampering with 11 and unlocking her power. That created a chain reaction in the upside down and the creation (or perhaps just alteration) of the monster. Hence, why 11 was able to destroy the monster. The two beings are linked.