My layman's understanding was that MWI doesn't postulate multiple universes. Rather, it posits that all particles are always in superpositions of states. What looks like a collapse to a single state is actually the particles under observation getting entangled with the particles that compose the observer: so, in the case of the double-slit experiment, you have a combined particle-observer system in a superposition of the states (particle goes through left slit / observer sees left detector activated) and (particle goes through right slit / observer sees right detector activated). Nothing has actually changed other than that entanglement: nothing is created or appears, certainly not an entire new universe.

ETA: like I say, I'm a layman, so I'd welcome any correction.