Gravity.

For short distances, the pull of the expansion of the universe is weak. The closer the 2 objects are, the weaker the expansion. I like to use a rubberband as an example because even if you can't visualize it, you can easily do it to see. Take a rubber band and cut it so it's not a loop. Place 3 dots on it, one "main" dot, one that's close to the main dot, and one that's far away. Now stretch the rubber band. You'll notice that the near dot doesn't move away from the main dot nearly as much as the far dot. The expansion of the universe works in the exact same way.

Inversely, gravity gets stronger the closer 2 objects are. Like magnets.

Andromeda, for example, is close enough to where the gravitational pull of our 2 galaxies is significantly greater than the expansion of the universe so our 2 galaxies will eventually collide in the distant future. However, anything outside our local group of galaxies is far enough away where the expansion of the universe is significantly stronger so we will NEVER collide with anything out there.