It has been said that humans are capable of reaching up to a billion light years away before expansion has taken those galaxies permanently out of reach without FTL methods. I aim to have a civilization use stellar engines from their home galaxy to reach these far out galaxies, build stellar engines on those stars and return them back to their home galaxy, red dwarfs will have the longevity of trillions of years for the long and slow journey back from the furthest reaches.
The largest known galaxy ic1101 has 100 trillion stars, which is massive compared to the trillion stars in Andromeda and around 400 or less billion stars in the Milky Way. In 4 billion years we will have the Andromeda and Triangulum galaxy merger with the Milky Way and by 100 billion years the local group will have merged into a super galaxy of under 2 trillion stars, at this time without out intervention there will be no new matter that can enter this super galaxy as all other galaxy groups and cluster would be out of reach and sight.
One of the major issues with bringing stars home with stellar engines is that the dark matter which is the bulk of the binding mass of the galaxy does not come with the star, neither does the black hole or inter stellar gas unless other methods are used to move or collect them.
At an estimate there are 500,000 trillion stars within one billion light years from Earth and if that many stars were tried to be contained within one galaxy it would be thousands to hundreds of thousands times larger than the largest known galaxies, And with a tiny fraction of the necessary dark matter to hold the super galaxy together it might not be possible to have so many stars as part of one system
We do have some ultra diffuse galaxies which are low or free from dark matter, they are the remnant of a galactic merger and clusters of stars that were thrown further out gravitate together without dark matter but these clusters or dwarf galaxies are small in comparison housing around 10 billion stars, similar to the size of the Triangulum galaxy
There are a few possibilities i had thought of to achieve this from lifting material to vastly increase the galactic centres black hole or even some sort of megastructure on immense proportions, possibly a simple arrangement like a globular cluster would work, there are proposed systems of matrioshka brains on clusters of red dwarfs but could that exist without lots of collisions on the massive scale i am after?
What methods that are within the laws of physics could an advanced species with stellar engine/megastructure and star lifting capabilities use to support a galaxy of 500,000 trillion stars (mostly red dwarfs) that has only a tiny fraction of the necessary amount of dark matter to bind it? Could it possibly exist as super giant globular cluster through a specific arrangement?