What additional areas can you unlock at the end of the game?

After you finish the story mode, you unlock

Mushroom Kingdom

I also got a notification saying I've unlocked another area for having over 150 power moons:

The Dark Side of the Moon

I now have about 380 power moons and the odyssey says I need around 100 more for something.

What other areas can I unlock and how many moons do I need to do so?

After you've gotten 500 power moons, you'll unlock the cleverly-named:

The Darker Side

Which, like the previous area you unlock, is super difficult. Instead of

boss refights

You will now have to do a bunch of difficult platforming.