The Hardest to find thing in the Universe?

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What is rarer than a shooting star?

Rarer than a diamond?

Rarer than any metal, any mineral, so rare that if you scan the entire earth, all six million billion billion kilos or 13,000,000,000,000,000,000,000,000 pounds of our planet, you would find only one ounce of it?

What is so rare it has never been seen directly, because if you could get enough of it together, it would self-vaporize from its own radioactive heat?