It's mostly dark matter. Apparently galaxies wouldn't be able to hold those shapes if they were merely made of the dust, gas, and stars we can see.

I always thought that frame-dragging, or something similar, could account for this, but I assume it's been ruled out already.

There is some debate regarding gravitational self-interaction. This[1] post gives an overview of a proposal by Deur.

Note though that while this is peer-reviwed work, it's still contentious.

[1]: https://astrobites.org/2020/08/17/the-alternative-to-dark-ma...

The only thing "dark matter" and "dark energy" have going for them is unfalsifiability for our type 0.5 civilization. Keep in mind that general relativity isn't actually even fully worked out for real systems.

> The only thing "dark matter" and "dark energy" have going for them is unfalsifiability for our type 0.5 civilization.

I don't think that's fair at all. Neither phenomenon is unfalsifiable.

By definition, dark matter an unknown gravitational effect. It's not fundamentally unfalsifiable, we just don't know what's causing it. The assumption is difficult-to-detect matter seems most likely because theories like MOND which suggest that "maybe gravity works differently from how we thought" are being investigated actively and just don't seem very likely so far.