This is because matter is incredibly, mind-bogglingly empty. An atom is like a miniature Solar System, with a tight nucleus playing the role of a Sun orbited by electrons like planets. But the nucleus is incredibly tiny compared with the orbits of the electrons. Tom Stoppard, the playwright, had the best image. He said, if the nucleus is like the altar of St Paul's cathedral, an electron is like a moth in the cathedral, one moment by the altar, the next by the dome. Imagine squeezing all the space out of an atom. Well, if you did that to all the atoms in all the people in the world, you could indeed fit the entire human race in the volume of a sugar cube.