The Sun contains more than ninety-nine point eight percent of all the mass in our solar system.
Why it is true
Jupiter and Saturn contain most of the remaining mass, while all other planets, moons, and asteroids make up a minuscule fraction of the total mass.
Source: Astrophysics Data
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.