Sponge-like animals that float through the universe, expending no energy, that filter through material whenever they find it. I'll make it more clear, but probably a combination of solar energy as well because that should be abundant when they pass close enough to a star. They only ever use energy to move when they are going to crash into something because they can detect when gravity is getting stronger and they push against it. They also only use energy for reproduction, probably asexual, maybe even production line esque, where they make more of themselves with hydrogen they collect by floating around.

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