Density is defined as the mass of an object per unit volume. The formula for calculating density is:

$$Density = \frac{Mass}{Volume}$$

In this case, the mass of the substance is 25 g and the volume is 100 cm³. Substituting these values into the formula, we get:

$$Density = \frac{25 g}{100 cm³} = 0.25 g/cm³$$

Therefore, the density of the substance is 0.25 g/cm³.