Well... almost all of physics is mathematical hacks to predict the future, insofar as it can be predicted.
The rest is cartoons and so-called "laws" which are only distinguishable from the math hacks in pedantic ways.

I didn't really mean to hypothesize that the disparity in angular momentum would be resolved with dark matter.
I only meant that they cannot possibly have enough data to make the claim they're making.

The %-age of even just the Milky Way that we have actually observed is relatively tiny. Much less the actual inner workings of distant galaxies.

My point was more to say that there is simply not enough data gathered by all humans throughout history to make claims about something on this scale.

The best they can claim is that given the laughably less than 1% of 1% of the universe we've observed, we find this trend... maybe that's indicative of large scale imbalance, or maybe its indicative that normal fluctuations should be expected and various regions of spacetime are expected to have different local angular momenta. Like whirlpools in a river. The total rotation in the river is expected to be 0, but at any location, it could be 1 way or the other. How big those volumes of spacetime should be we don't really know. Or at least, I don't know how to define that.

As you said, our entire observable universe may be only a tiny fraction of all the contiguous spacetime - not even counting "parallel" universes, just the same universe, but so far away as to be unobservable... we don't know how big those pockets "should" be.