What property or properties of gases can you point to support the assumption that most
of the volume in a gas is empty space?

Explanation:

One of the assumptions of the kinetic molecular theory is that the gas particles are way smaller than the separtion between them and, in consequence, most of the volume in a gas is empty space.

This condition (that most of the volume in a gas is empty space) explains why the gases can be easily compressed: since there are vast unoccupied spaces when the pressure is increased, the particles can approach each other with which the total volume of the gas decreases.

Expandibility, the property of expanding, is the ability that gases have to occupy the entire space of the container in which they are located. The particles then will get separated leaving most of the space empty.

The fact that most of the volume in a gas is empty space means that the volume for a certain amount of particles is much larger than the volume that the same number of particles in a solid will occupy, driving to much lower densities.