Probability space

The Everything Sea
Other dimensions are somewhat easy to understand- they have physical locations. Locations in four or more dimensions, granted, but physical locations nonetheless. Flatland analogies can be used.

Probability space isn't like that.

Probability space is potential, lost or waiting, embodied. In the mass that is probability space, every possibility exists. Every path history could have taken. They don't exist as places (fortunately for those with dimension powers, who would prefer not to be pestered by infinite versions of themselves), they just exist as potential.

Unless someone collapses the waveform.

Crystallising probability space
Within probability space, it's possible to "look for" a certain possibility. It's a strange process, mostly kept to machines, although some supers do it instinictually. The closer a timeline is to ours, the easier it is to find. Once found, it becomes real. People appear, and think they lived there their whole lives. An entire alternate history, created from nothing.

Note: It becomes completely real. Unlike a shadow dimension, it isn't bound to our history, and can diverge massively. In almost all cases, minor changes mean alternate timelines have no counterparts to people born after their "creation".

This is a major problem. However, it's also a major opportunity. The new world is a real place, and things can be taken there and back. Most of these timelines collapse back into quantum foam soon (arguably mass murder, another reason this isn't done lightly), but some remain indefinitely.

So far around 30 timelines permanently exist in probability space, with three (one where an alien virus ravaged humanity in the 40s, one where excess magic made the world mildly subjective in the 1960s, and one where powerful psychic entities gave superpowers to thousands of random people in the year 2000) having made contact with our world. None of these worlds have any counterparts to our superhumans.

Probability space and the future.
The future exists in probability space. However, it's not quite the same way. Unlike people in alternate timelines, who don't have autonomy until their world crystalises, people from the future (from multiple contradictory futures, indeed) can and do come back in time to our timeline. Also, travelling to a future doesn't "crystalise" it as it would an alternate timeline. That future is no more or less likely to come to pass then one not visitied.

This seems to imply the future exists in the opposite way to other bits of probability space- every possible future exists, until one becomes real and the others collapse into probability space as alternate unreal timelines.

The exact implications of this cosmological structure are perhaps best left unexplored.