The atom is mostly empty space, if we removed that space all of humanity would fit in a sugar cube.
Why it is true
The nucleus of an atom is incredibly dense, but tiny compared to the electron cloud. If we compressed all the empty space in the atoms of every person on Earth, the resulting mass would be tiny.
Source: CERN
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.