I mean, idk about dark matter, it has always felt like a mathematical hack to explain gravity as we observe it. So what, now it's also a hack to explain angular momentum as we observe it?

Dark matter is of course one possible explanation. But it's no different to anything else unobservable, such as a parallel universe, or Planck-scale events, or something happening in the first fraction of a second after the big bang. Personally the most likely solution to me is that our observable universe is, essentially, too small for us to draw conclusions. If we "zoom out" then balance is restored. That's my intuition. But maybe you're right, maybe dark matter is real and has angular momentum.

One potential explanation I saw for dark matter was a "Planck relic", basically the remnants of evaporated black holes. The idea is that a black hole cannot fully evaporate since energy is quantised, so when a black hole reaches the lowest possible mass (1 fundamental unit of mass/energy), it cannot radiate more energy. These "Planck relics" would be far too small for us to observe directly, but if there's enough of them we could certainly observe their gravitational influence on other objects.

I like that idea. Maybe these Planck relics exist and have angular momentum, if so then your idea that dark matter is where the missing angular momentum is would potentially be correct.