Kal Dano was a scientist from the 27th century who invented the Tox Uthat, a quantum phase inhibitor that could halt all nuclear fusion within a star.
When two Vorgon criminals, Ajur and Boratus, attempted to steal the Uthat, Dano traveled back in time to the 22nd century and hid it on the planet Risa, where it would remain undiscovered until the year 2366. (TNG: "Captain's Holiday")