I need help about a certain phrase. I couldn't grasp the exact meaning of "exist at all" in the following sentence:

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> This is the basis of nuclear fission reactors, where unstable nuclei of very unstable elements such as uranium and plutonium are encouraged to split into lighter elements, with energy being released. *Those heavy elements only exist at all*, and the energy is only available to be released, *because energy was put in to their manufacture* in the dying explosion of a star (or stars) that started out with at least eight to ten times the mass of our sun. (*Alone in the Universe: Why Our Planet Is Unique* by John Gribbin)
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(The sentence is about uranium and plutonium, by the way.)

Does it mean that they exist in small amounts?