A galaxy without dark matter ?

This looks extremely interesting. Annoyingly, the link to the paper is broken (and my institute doesn't have a subscription to Nature anyway). I guess it'll be on arXiv soon.

DF2 is known as an “ultra-diffuse” or “ghost” galaxy, an extremely low-density variety, recognisable due to its large size and faint appearance. However, this one is “an oddity, even among this unusual class of galaxy”, according to Shany Danieli, a Yale University graduate student who contributed to its discovery.

The astronomers realised something about DF2 was amiss when telescope observations revealed that 10 clusters of stars within it were moving far slower than would normally be expected. The velocities of stars and other objects in faraway galaxies can be used to measure their individual masses.

By performing these calculations, the research team found that all the mass in the galaxy could be attributed to the visible stars, gas and dust. There was essentially no remaining room in this galaxy for dark matter.

Just as we should be cautious about claiming some other UDGs are extremely dark matter rich based on observations of only a handful of globular clusters, so we should be equally cautious about finding galaxies with no dark matter by the same method.

Counterintuitively, Professor Van Dokkum and his colleagues suggest the lack of dark matter in DF2 is actually good evidence for its existence. While this substance plays a central role in our understanding of the universe, its intangible nature means alternate theories have been suggested to account for the gap in scientific understanding of what is currently known as dark matter.

These theories consider the dark matter signature that astronomers measure to be an unavoidable consequence of ordinary matter. Therefore, the existence of a galaxy that has lots of matter, but no dark matter, suggests dark matter does indeed exist elsewhere as a substance in its own right.

I've long suggested that if we find a galaxy with a Keplerian rotation curve (as standard Newtonian gravity predicts), both modified gravity theories and dark matter would be screwed. However, with 10 data points we won't get a nice rotation curve, just a total mass value. And there are a whole bunch of caveats to this anyway, so I'm reserving judgement and further commentary until I've read the paper.

https://www.independent.co.uk/news/science/distant-galaxy-dark-matter-universe-understanding-theories-wrong-space-yale-a8277951.html