I agree, I'm not a fan of calling it "Dark Matter." It's only called "dark" because we don't understand it. You wouldn't call it "dark" if you understood it.
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I noticed this with a Gasopod that I released, but not fish. However, I've released another Gasopod and Jellyray near my base and they have yet to despawn. I'm starting to think they might just migrate away to where there is food.