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Lets take a civilization that has surpassed the type III on the Kardashev scale. It has the following assumptions:

• It harnesses the energy of entire Galaxies,


• its populace did not transcend. If you need a reason: passed a certain point of intelligence biological life sees no reason to live and dies off so they avoid getting smart enough.


• The civilization has found a material strong enough to build space elevators and Dyson Spheres.


• The civilization can create matter and energy from nothing, otherwise all natural laws are still "normal".

The civilization expands across the galaxy and galaxies around it. As the population grows the civilization decides to fill the universe with matter to make as much living space as possible. The question is, how far can they go?

With such powers it would be easy to fill the universe with Dyson Spheres. But could they go further? I imagined a galaxy where the planets/Dyson Spheres stop rotating (or have a rotating connection at the poles if the rotation is important) and solar systems are connected with "bridges" that can house trillions upon trillions. Eventually Gravity will be the limiting factor, as mass would cause the structure to collapse in on itself to form a new planet and if too much matter is in one place it'll eventually form Black holes. On the other hand there's also the expansion of the universe to consider and what it would do to such a Galaxy...

So what would the fullest most liveable Galaxy look like, without it collapsing in on itself or breaking apart?

If your civilization can create matter and energy out of nothing (violating conservation of mass and mass-energy equivalence), then your limit is immaterial. The universe's expansion is driven by energy. If you can create infinite energy, you can expand the universe to an unlimited degree, because the added mass can be counterbalanced.

Assuming your materials are as nigh-magical as they appear to be, then take the available volume of the universe ($4 times 10^{80} m^3$), divide by the space of a luxury apartment + amenities, which for convenience we'll set to ${1000}m^3$, and you have housing for $10^{77}$ humans, plus three times that space to grow food for them, assuming you can't just conjure that out of thin air as well. Gravitational collapse is not a problem, since stresses will be identical in all directions and the entirety of the universe will remain mostly empty space. Your only real problem will be getting rid of heat, but if you can create matter and energy, that seems like a bagatelle.

Effectively an infinite apartment building. There's probably a horror movie like that.

Edit: It's worth pointing out that we're assuming your humans have solved FTL travel, because if they haven't, their magical abilities aside, they're never going to populate even a small corner of the universe.

The civilization can create matter and energy from nothing

If they can pull out stuff and energy from their aethers impunely, they can create a singularities that expand at light-speed if they so wish.

Simply put, if they use technology to make something out of nothing, they can create ever more machines that do it. The amount of mass they can create grows exponentially. The only limiting factor to what they can do would be the speed of light, and they can go past that if they develop the means for FTL travel.

So the amount of livable space they can make is up to what they call livable. Some people in Asia find it normal to sleep in pod hotels, whereas I would move to Mars without a second thought if I knew I could be the only person there.

If your people really like each other, then as long as gravity is not enough to knock them out you can keep expanding. Food is not a problem - you can just conjure stuff out of nothing, and then turn excrements into EM energy and send it towards the edge of the observable universe.

And who the Hades builds Dyson machinery when you have energy out of nothing?

It would seem to stand to reason that the goal ought not to be to build one single super-sized galaxy, but a series of regular or slightly larger than normal sized galaxies that exist together (yet independently) in a super cluster.

Superclusters are so large that the galaxies continue spreading outward with the universe. To illustrate, if you imagine, say for example, ten galaxies in a supercluster represented as ten dots made by a marker on a partially blown up balloon. As the balloon (the universe) is blown up further (expands) the dots (galaxies) will spread away from each other. During this time, you could be busily drawing the dots larger (building the galaxies larger), seemingly infinitely. The larger the dots (galaxies) become on the balloon (universe), the more over all space is taken up. But you do so in such a way that the percentage of the balloon (universe) that is filled with these dots (your supercluster) never actually increases or decreases, as it keeps pace with the expansion of the balloon (universe). Follow me?

In short, your answer is probably "a super cluster, adjusted for inflation"