What Lies in the Multiverse

• Chapter: 5
• Location: on the lower level of the screen, not far from the "H" shaped void
• World: Light (as opposed to the Dark one)
• Requirement: talk to the fisherman while the goose steals his sandwich and then go after the goose
• Story-related? No; if missed on your first playthrough, you can use chapter select

After beating the "H" shaped void puzzle and opening a locked door, you'll have a brief conversation with Lise. After this conversation, Everett and Nash will have an altercation on the top of a bridge, on the lower level of the screen. That's where you have to go.

Proceed to the right and go down to the lower level on the very first opportunity (over a long bridge, which will disappear when you switch worlds). Now proceed to the left solving a few puzzles until you find the fisherman.

Talk to the fisherman, during which a goose will steal a sandwich from his basket. After conversation is over, go left to find the goose.

For a detailed walkthrough, check this video by YouTube user Carrot Helper (all credit to them):