The Dark Museum is an area accessible from Geometry World through the door in the last block of the row of blocks.
Features
This area is much like the ordinary Museum with a similar layout, however it has maroon carpets, almost rustic-looking walls that sometimes have images on them, larger doorways and tables and chairs dotted around.
This area is also completely dark and is impossible to navigate through without the Lantern Effect, and even then you can only see so far in front of you, so you may have to keep pressing the Action button in order to get a good look at what's around you.
You can enter this area from Japan Town through the red mailbox, however you will be stuck on a pedestal with no way to get off again aside from teleporting back to Japan Town.
By going up into the doorway pictured (it is near the entrance), going left to the next doorway, and travelling up and left in the same room, you should find a yellow ticket booth portal. By entering the ticket booth, you will be teleported to Flying Fish World. If you interact with the ticket booth here, it will send you instead to the ordinary Museum.