Mike Lorrey asks whether the many-worlds interpretation of quantum mechanics would violate conservation of mass (and energy), since new worlds are being "created" all the time.
The short answer is no, because that's not how conservation of mass is defined. In quantum terms, conservation laws apply to the wave function, and in many-worlds, there is only one wave function which describes the "multiverse". What happens is that the wave function can be interpreted mathematically as having multiple components, all of which behave independently of each other, and which look like worlds to the self-aware mathematical structures (us) which they include.
"Creating" a new world actually involves the wave function evolving to a state where there is additional mathematical structure, such that more components of the function can be identified which don't interfere with other components. This looks, to the inhabitants, like splitting of the world so that there are now two worlds which don't interact. But mathematically it is just a matter of evolving a more complex wave function which describes the global universe of all the worlds, and there are no violations of conservation laws.