According to the universal law of gravitation, there is a force of between any two objects in the universe.

Attraction

Explanation:

According to the Newton's law of gravitation, the two bodies attract each other with a force which is directly proportional to the product of masses of two bodies and inversely proportional to the square of distance between them.

Where, g is called universal gravitational constant and the value of G is 6.67 x 10^-11 Nm^2 / kg^2.

increase masses, decrease separation

3. dark matter

empty space could also be a simple vacuum