galaxy A is on one side of the universe
galaxy B is on the other side of the universe
galaxy C is somewhere roughly mid way between A and B, and contains star 1 and star 2

star 1 and star 2 exert a gravitational pull on each other, meaning that space is curved

the curvature of space by star 1 and star 2 means that galaxy A and B are now closer to each other despite not communicating