While sci-fi portrayals of the asteroid belt demonstrate it as a dense region of asteroids lying between Mars and Jupiter, this is strictly not the case for the actual asteroid belt in our solar system.  

If all asteroids in the solar system were to be combined into one body, the total mass of that body would be around four per cent of the mass of the moon – with about half of this mass being concentrated in four of the major asteroids, namely Vesta, Pallas, Ceres and Hygiea. Indeed, most of the asteroid belt is just empty space, with an estimated average distance of about one million kilometres between asteroids!