Superman, the Martian Manhunter, and Wonder Woman are off fighting whatever Herald(s) preceded Galactus, and the robotic Punisher that once took on the FF is now busy holding back Hawkman, the Atom, and Zatanna.

The Flash ran through time and space to get the Nullifier and he's too tired to do much of anything else at the moment. Clearly Batman has the best poker face to make Galactus believe he'd actually use it, so he gets sent up to speak to the big G.

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That turns it into something quite a bit more than the oft-heard "Batman could defeat Galactus" scenario, doesn't it?