“Our knowledge of physics only takes us back so far. Before this instant of cosmic time, all the laws of physics or chemistry are as evanescent as rings of smoke.” -Joe Silk
When people use the word theory colloquially, they use it to mean an “idea” or a “possibility” that could conceivably be at play. But a scientific theory has a much more stringent set of things it must accomplish: it must encompass all the successes of the previously leading theory, it must make successful predictions for phenomena that the leading theory cannot make, and it must predict additional, novel phenomena that can be either validated or refuted.
So, is the Multiverse a scientific theory? It arises as a consequence of a scientific theory, but it fails on the grounds of making successful and novel predictions. While it might someday rise to the status of a scientific theory, it isn’t there today.