[astro123]

> It seems significant that the lens galaxy has little dark matter

I don't see that in the article? They centered on galaxy clusters which contain huge amounts of dark matter. DM is needed to account for lensing. There isn't enough mass in the stuff we can see to account for the lensing signal.

> How is DM supposed to cool, anyway

It can't! The way that normal matter cools is by radiating (giving off light). Dark matter doesn't radiate (if it did, we would be able to see it). So, while normal matter cools and settles to the center of the gravity well, dark matter doesn't and (as you say) forms physically larger structures.

[ncmncm]

I am referring to [0]. Tl;dr: Galaxy rotation curves are accounted for by ordinary General Relativity. At least for some. My comment turns out to refer to another article [1] linked from TFA, referring to a different sighting, that says "the lensing galaxy which hosts the radio source contains an unexpectedly low fraction of mysterious dark matter".

Usually we hear about lensing by galaxy clusters, not individual galaxies. If galaxies are short on DM, but clusters aren't, that seems to say something about how DM is distributed.

[0] <https://www.sciencedaily.com/releases/2021/03/210304145458.h...>

[1] <https://phys.org/news/2014-06-herschel-key-discovery-spectac...>