Why must the universe be an isolated system to obey the Law of Conservation of Mass?

(see below)

The trivial answer is:

If the universe were not an isolated system mass could be exported or imported from somewhere outside the universe thus either decreasing or increasing the amount of mass in the universe.

Problems with this answer:

[1] Couldn't the Conservation of Mass only apply to the meta-universe?

[2] Is the Law of Conservation of Mass completely accurate? What about mass- transformations? Where did all this mass come from in the first place?