Hydrogen is the rarest element in the universe.
Why it is true
Hydrogen is the most abundant element in the universe, representing about 75 percent of visible matter. It forms the majority of stars and is present in gigantic clouds of interstellar gas. Heavier elements are much rarer.
Source: Nature
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.