While pushing toward even stranger universes, I discovered another design glitch. So, as soon as dark matter reaches extreme levels (in excess of 100), converting those galaxies into gigastructures becomes unfeasible due to insane stone requirements.
What do I mean?
As I noticed, max. system size is 50 planets. At my level, the largest planets were between 1.5 and 1.7 Mkm, 2 to 4 of those per system. Based on that assumption, I would have to have PK'd some thousand systems (manually, as there is no auto-PK) to be able to convert an 1000-DM galaxy. Sure, it would have been a powerhouse (meaning my main source of minerals/research wouldn't have been imbalanced terraformed planets or megastructures anymore), but who would do so much manual work?
Besides the necessary late-game rebalancing, a stone-/metal-producing gigastructure would be nice - or the possibility to buy stone. I mean, you can already buy any amount of any material at a fixed price (which is quite unrealistic), so why not stone?