"simulated the universe as a collection of 1,000 particles that interact with one another only by gravity, representing the galaxies and stars that float around

"simulated the universe as a collection of 1,000 particles that interact with one another only by gravity, representing the galaxies and stars that float around the cosmos. The researchers found that regardless of starting positions and velocities, at some point the particles inevitably find themselves clustered together in a ball before dispersing again" Wait, what? That conclusion doesn't make any sense to me.