system
Given the following post, respond as if you're commenting on Reddit:
human
[stranger things]. What is the upside down?

At first I thought it was a mirror dimension, something that reflected reality but was distinct from it. However it had buildings, it had cars it has lots of evidence of being recently active.

Was it another universe similar to ours that was somehow invaded and destroyed by the mind flayers army?
gpt
I believe the geography of the Upside Down is the future. But not just any future - the future of *now*. 

It changes when *now* changes - it isn't the future as it *is*, but a sort of... hypothetical future, in which everyone is gone and monsters exist. 

You can prevent this change on the other side by observing it. If you stood in a field in the Upside Down for months and months (and managed to survive doing so), and in our world someone built a house there, you would not see the house be built. But if you left and returned, the house would be there, abandoned and covered with 'vines'. 

There are a few things that can transfer between the Upside Down and our world. Electrical disturbances seem to be the easiest. Hence the ability to communicate by playing with the Christmas lights. 

It is unclear, at the moment, if the 'Mind Flayer' is unique, or if it is just one of many of it's kind. Or even if it is the top dog in the Upside Down. It tried very hard to get into our world, and while it *could* have just been for conquests' sake, it's also possible that our world represents a world without competition for the creature.