All matter has density becuase all matter takes up space. When you wave your hand infront of yourself you feel wind. if air did not take up space it would all collapse into a black hole becuase their would be no resistance.
Wolf gang polli made the polli exlution princible, it says that all fermion particles take up space and can not be in the same place at the same time. This is why your legs hold up, your Piano doesent fall through the ground. if this wassent true everything would collaps into a black hole
