Here's why:

* Gravity is a universal force: Every object in the universe has gravity, which is a force of attraction between any two objects with mass. The more massive an object, the stronger its gravitational pull.

* Stars are massive: Stars are incredibly massive objects, containing enormous amounts of matter. This means they have a very strong gravitational pull.

* Gravity attracts: The gravity of each star in a galaxy pulls on all the other stars. This mutual attraction is what holds galaxies together, preventing them from flying apart.

* Balance of forces: While gravity pulls stars together, their own internal pressure and the momentum of their motion help prevent them from collapsing into one another. It's a delicate balance!

So, while stars are spread out across vast distances, it's the force of gravity that binds them together in galaxies and clusters, creating the beautiful structures we observe in the universe.