The sideway universe was a universe the souls of the characters created together after their deaths to be reunited one last time before they all "moved on" (with a solid implication through the scene of Christian opening the doors to the light that when they moved on to the real afterlife they all became one with the Light, which can be interpreted in different ways... Mine is that the Light turns out to be the sum of all souls, so among other things it's also the "afterlife" and probably the sum of all souls=God as well, and it's the energy pool that gets recycled to give birth to new souls as well - while the darkness the Light protects from would be the sum of all human evil (incl. greed, selfishness etc.) the souls shed when entering the Light.

The impression it left me was that Hugo was the instigator of the sideways, that he's the one who could talk to the dead and make them all wait for the last one to die before they had this "activity" to find each other again (the most meaningful experience of their lives was their encounters and what they lived together) and then they all moved on (that last one to die most likely was Hugo himself, thousands of years later maybe).

All those who were not ready to "move on" to the afterlife have remained behind for now (Ben, Ana Lucia etc. and earlier on the Island the "whisperers" would couldn't yet move on).