Why do the densities of most substances increase when temperature decreases?

This is because temperatures determine the kinetic energy of molecules of a substance, At lower temperatures the molecules have low kinetic energy hence the distance between molecules is not as large as when the kinetic energy is higher (because the molecules bombard less and with less kinetic energy). This means the substance can pack more molecules per volume at lower temperatures. The more the molecules per volume the higher the density.

Answer - Volume's proportionality to temperature is direct.

Explanation-

As we know density is the mass of an object per unit volume of that object it depends on the volume and we also know that volume is dependent directly on temperature does. When temperature of something increases, then the volume of that object also increases. Because on heating the molecules start moving faster and faster movement of molecules need more space.

Hence the opposite happens when temperature decreases the volume and volume being inversely proportional to density causes and the density to increase in kinetic energy.