This might be a completely stupid question but I would like to understand: In the many-worlds interpretation (MWI) if the universe splits at some point (wave function collapse), does it mean that all the matter in it duplicates? How would that work?
The key is to remember that matter is a wave, or at least its interaction with other stuff is governed by a wavefunction, and it's easy for waves to split. Diffraction, partial reflection, even just watching a wavefront hit a narrow obstacle and diverge, can give you some intuition about how MWI posits that wavefunctions can split and yield multiple configurations (worlds). Though it's not exactly like how water waves work of course, just food for intuition.