The Alternate World

After defeating Lusamine and rescuing her from Ultra Space, a player can return to the Altar on Poni Island to find that there is still wormhole remnants. This is a small wormhole that the player can go through, but they must have either Solgaleo or Lunala in their party with them. If you have Solgaleo, the wormhole only opens at night; if you have Lunala the wormhole only opens up during the day. This is not Version exclusive: if you have a Lunala in Pokemon Sun, you still must go during the day, and so the same with Solgaleo in either version but at night.

The Alternate World essentially inverts the Time within your game, where day becomes night and night becomes day, where AM becomes PM and PM becomes AM. This essentially makes the overworld of Pokemon Sun be like the overworld of Pokemon Moon, and vice-versa. It allows for a player to do certain time based event, such as search for all thee time-based Zygarde Cell Locations. The Altars on Poni Island also become their counterparts, with the Lake of the Sunne and Lake of the Moone switching as well. However, despite this, the Pokemon set to your given Version remain the same: Rockruff and Cosmoem will still evolve into their version specific evolutions, and you'll still only find Version Exclusive Pokemon for your given game in the Alternate World as well.

On top of all of this, you can only find the only Cosmog in your game within the Alternate World. This will be at either the Lake of Sunne or Lake of Moone, depending on your Version and what it becomes in the Alternate World, where Cosmog will appear and wish to join your team.