The Hardest Thing To Find In The Universe

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 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? What is this stuff that can't be seen or found?