All the matter that makes up the human race could fit in a sugar cube

Despite what our intuition about atoms may be, they are 99.9999999999999% empty space. As Tom Stoppard put it: “Make a fist, and if your fist is as big as the nucleus of an atom, then the atom is as big as St Paul’s, and if it happens to be a hydrogen atom, then it has a single electron flitting about like a moth in an empty cathedral, now by the dome, now by the altar." 

If you were to force all of the matter together, thus removing all of the empty space within and between them, a block the size of a single sugar cube would weigh an incredible five billion tons! (To put this into perspective, this is about ten times the weight of all humans currently living.) 

Another way to look at the astonishingly small size of atoms is with this clever analogy. If you were to take a regular sized apple and magnify it so that it was as large as the earth, then each individual atom in the original apple would be about the size of the entire apple within the new earth-sized apple.