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The Lord of Admirals says that the Gravemind who sent him was responsible for the destruction of other galaxies, said this to Librarian. So either gravemind destroyed the galaxies utterly or he infected them.
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I don't recall that. Can you do a direct quote and citation?
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Considering that the flood initially came from beyond the edge of the galaxy (if memory serves) it seems likely that at the very least wherever they can from is probably an infested hell hole.
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They originated from the Large Magellanic Cloud. Curiously enough, it is not infested in the slightest, and any remaining traces of the Precursors there was destroyed when Omega Halo was fired at the LMC.