Step 1: Recall the law.
The question describes Newton's law of universal gravitation, the pull between any two masses in the universe.
Step 2: Write the dependence on mass.
The force grows with the product of the two masses \(m_1 m_2\), so bigger masses attract more strongly.
Step 3: Write the dependence on distance.
The force gets weaker as the masses move apart, and it falls off as the square of the separation.
Step 4: Put it as a formula.
\[ F = G\,\frac{m_1 m_2}{r^{2}} \] where \(r\) is the distance between the centres.
Step 5: Read the inverse part.
Since \(r\) appears as \(r^{2}\) in the denominator, the force is inversely proportional to the square of the distance.
Step 6: Choose the answer.
The blank is the square of the distance between them.
\[ \boxed{\text{Square of the distance between them}} \]