What Lies in the Multiverse

• Chapter: 7
• World: Dark (as opposed to the Light one)
• Requirement: switch worlds so that the Kid "falls" up, off the screen
• Story-related? No; if missed on your first playthrough, you can use chapter select

It is very likely that you'll get this one by playing normally.

The "dark" world in this level has an inverted gravity system, so switching worlds will simply make you "fall" up.

Therefore, from the moment you gain the ability of switching worlds onwards, simply switch by using in a place where you don't have a roof.