[t8sr]

Definitionally, yes. It’s inert but lenses light around it.

The paper is more about the technical achievement of detecting it, IIUC. It’s not the first dark matter inference we’ve had, and doesn’t really tell us anything new about the stuff.

[daxfohl]

It challenges warm dark matter and ultralight dark matter theories because they'd be less likely to clump into something so small. Similarly MOND would have trouble explaining a completely isolated chunk of it at this size (any baryonic matter trapped in a region this small would almost certainly emit enough light to detect).

[t8sr]

I’m admittedly a few years out of date in this, but weren’t those already kinda ruled out? I’ve never met anyone who took MOND seriously - it looks like it’s a pet project of a small number of people who cite each other, and people in different subfields have always been saying it doesn’t work for them (diffuse galaxies, etc.).

I know the current models favor cold DM, I thought the hot DM model was abandoned already when it became clear that clusters of any size exist?

[mr_mitm]

Your assessment is spot on, but for whatever reason it's extremely popular with amateur physicists. The HN crowd likes it a lot, too. Almost every thread about dark matter has at least one comment that goes like "I'm not a physicist but dark matter always seemed like a cop out to me and will go the way of the luminiferous aether". I'm surprised that we're not seeing them here, perhaps because MOND can't explain this.

[daxfohl]

And yes, hot dark matter has largely been ruled out, but there are still extensions into warm dark matter and ultralight dark matter that seem more manufactured but are still plausible. The observations in this paper creates some additional challenges for those theories.

But yes, CDM is what most researchers expect, by a large margin.