On cosmological scales (that is, on scales greater than 100 Mpc or so), the
dominant force determining the evolution of the universe is gravity. The weak
and strong nuclear forces are short-range forces; the weak force is effective
only on scales of ` w ∼ 10 −18 m or less, and the strong force on scales of ` s ∼
10 −15 m or less. Both gravity and electromagnetism are long range forces. On
small scales, gravity is negligibly small compared to electromagnetic forces;
for instance, the electrostatic repulsion between a pair of protons is larger by
a factor ∼ 10 36 than the gravitational attraction between them. However, on
large scales, the universe is electrically neutral, so there are no electrostatic
forces on large scales. Moreover, intergalactic magnetic fields are sufficiently
small that magnetic forces are also negligibly tiny on cosmological scales.
Ironically then, gravity – the weakest of all forces from a particle physics
standpoint – is the force which determines the evolution of the universe on
large scales.