This question started me thinking about the nature of the mirror universe.
Just what kind of universe is the mirror universe in Star Trek? I can think of three possibilities (and realize there may be more):
Is it an actual mirror universe, or is it a mirror image that was created with our universe with some kind of symmetry to us, so it'll always be a dark reflection of our universe?
Is it simply an alternate universe that happens to have a lot of similarities to our own?
Is it an alternate timeline that forked at some point along the way when one person made a decision and things split on a quantum level?
And if the last one, then are all the different trips to the mirror universe, from Enterprise through Deep Space Nine to the same alternate timeline, or to different ones? If they all went to the same one, why to just that one?