A collection of gravitationally bound particles will be attracted together. But as they come together, their velocities increase. They’ll come to some close approachand generally whizz right past each other, sailing outward again before being attracted together again. Even if two particles smash head on, they’ll likely just ricochet in a perfectly elastic collision (tough to know for certain - all other examples of particles clashing together involve other forces. A purely gravitational collision with no other forces involved we can only speculate about.) A head on collision in a diffuse cloud of infinitesimal particles is vanishingly rare in any case - much more likely are two particles passing by, maybe having their paths deflect a little, but not an actual collision. Some particles may gain some velocity in interactions, others lose it, but the total potential and kinetic energy of the system is constant. The volume may decrease, but the average velocity would then increase, which will lead to the volume increasing again.
This is described mathematically as Virial Theorem, and is used to describe the dynamics of stars in galaxies, or the large-scale movement of galaxies across the cosmos. Stars are another situation where the particles (stars) are almost infinitesimally small compared to the interstellar distances, so actual collisions are vanishingly rare, but gravitational interactions still take place. The result? We have galaxies, with lots of stars swirling around, not one single giant black hole. The clumping and structure we have in galaxies is due to EM interactions between atoms, allowing for the gravitational energy of the system to change.