Galaxies still collide.

The expansion of the universe doesn't mean that galaxies cannot collide. There are at least two galaxies. That is coming to the Milky Way. The Large Magellanic Cloud will impact the Milky Way in 2,4 billion years. And Andromeda galaxy collides with the Milky Way after 4,4 billion years. 

That thing means that there are two versions of the universe. The thing is that the galaxies in the local clusters turn into one entirety. And then. Galaxies in superclusters turn into one entirety. Because their mutual gravity wins dark energy.

There are two geometries in the universe. Local and global geometry. Local geometry is the thing. That we see in large galaxy clusters. 

Global geometry means. The universe's geometry as an entirety. 

The fact is that in local geometry gravity always wins. But on a global scale, the dark energy wins. The large-scale models show that the universe expands. However, the small-scale model can be different. And in small-scale models the galaxies in local galaxy clusters or groups will come together. In local galaxy clusters, the internal gravity of those clusters wins the expansion and that causes the galaxies will collide. All galaxies pull material from around them. 

That increases their mass. And expands the galaxy's gravity field.  When galaxies collide. That new structure's mass is the same as both galaxies' mass. 

That increases the size of the gravity field or gravity pool. Gravity is interaction and the same way distant galaxies pull the Milky Way and Andromeda galaxies to them, as Milky Way and Andromeda pull those galaxies to our direction. 

Material is not homogenously spread all over the universe. Things like the cosmic web make modeling those kinds of systems very difficult. 

There are interactions between galaxies, galaxy clusters, and globular clusters. And the fields in those cosmic web structures. The gravity effect forms those complicated structures in the universe. Gravity is the strongest interaction at long distances. 

But that is not the only interaction in the universe. There are other interactions like electromagnetism. Those interactions are losing to gravity. But they have effects in short distances.  

That means that things like gravitational effects are not the same everywhere. Galaxies are in clusters. Those clusters are like bubbles. 

In those bubbles, there are local clusters. And superclusters, groups of local clusters. The mutual gravity in those clusters pulls galaxies together. 

Those bubbles form galaxy clusters. In galaxy clusters, the internal gravity of those structures pulls their galaxies together. First, the local clusters melt into large galaxies. And finally, those superclusters melt into single giant galaxies. The thing that the universe's expansion and the decrease of the energy level causes is that. Entropy in the system rises. That causes two effects. 

First, the entropy disturbs information more than in the young universe. But. Otherwise the energy levels in those fields are lower. That means the gravity pools around the galaxies turn larger. The gravitational fields affect a longer distance because there are not so powerful disturbing fields. The gravity centers are larger but the space between those large galaxies will be cold and dark. 

The energy level difference in and outside those galaxies is higher than it is in the modern universe. That causes situations where material in galaxies vaporizes more than in the modern universe. Galaxies send radiation stronger than in the modern universe. But there is less material and radiation between galaxies. 

That tells that there was some kind of turbulence. Or something that was denied. The material spread homogeneously over the universe.  There were some kind of gravity centers. That could be primordial black holes. That started the form of those galactic clusters. 

https://bigthink.com/starts-with-a-bang/why-galaxies-still-collide-expanding-universe/

https://www.sci.news/astronomy/large-magellanic-cloud-milky-way-collision-06788.html

https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision

https://www.sciencealert.com/a-supermassive-black-hole-is-on-a-collision-course-with-the-milky-way