God is so quiet

[SteveII]

No, there is no logical reason that the physical universe/cosmos/multiverse exists necessarily. 'Necessarily' means "could not have been otherwise". We can all conceive of a state of affairs (a possible world) where nothing at all exists and another state of affairs where they do.

And yet I can conceive of God not existing in one possible world yet existing in another. So that's not how you argue against metaphysical necessity of an entity.

Suppose the cosmos is all that there can possibly be, with all possibilities actualized (via multiple universes and multiverses), then going with modal logic, there is no possible world where such a cosmos is not a thing.

Enough special pleading.

You are conceiving a logical contradiction. If God exists in one possible world, then he exists in all possible worlds. If he fails to exist in one possible world, he does not exist in any possible worlds. These are two sets that are bound together by logic. All or nothing. You can't mix them without creating a contradiction. That's why I phrased it "If God exists, he exists necessarily". 

Regarding your cosmos, we can still logically conceive of a possible world where there is not anything. It is logically compatible with a possible world where it does exist. Therefore the cosmos is not a necessary entity (if it does exist it could not have failed to exist).