Recently, a few people on X pointed out to me a paper by a Chinese group (Lui et al., 2025) who have been monitoring neutral hydrogen outside the Milky Way disc and noticed what they call a compact gas clump, named poetically: AC G185.0-11.5. It is a swirling mass of hydrogen that somehow stays together. The mainstream of course proposed adding as much dark matter as needed to keep it together within the remit of general relativity, and called it a 'dark galaxy' (there are others like this, such as Dragonfly 44), but we know how to do testable physics, so we can do better.
From a paper I wrote in 2012 (see the reference), quantised inertia (QI) predicts that the velocity of the material at the edge of a system at very low acceleration has to be just large enough to keep the acceleration above the cosmic minimum (2c^2/Cosmic_scale), so it will be
v = (2GMc^2/Cosmic_scale)^0.25
So let's see if QI can predict this newly-observed system (you know it will, or why would I be writing this with such a confident air!). Its gas and dust mass was given in Liu et al. (2025) as ranging from a possible 3x10^7 to 4.7x10^8 solar masses, and the cosmic (Hubble) scale is 8.8x10^26 m, so using the QI equation above, the predicted orbital speed has a low of 30 km/s and a high of 58.9 km/s, so let's say 44.5 +/- 14.5 km/s.
So what do Lui and crew say is the observed rotational speed? Drum-roll... It is 42.2 +/- 2 km/s. QI predicts it well. It correctly predicts Dragonfly 44 as well (I wrote a blog on that one in 2016). I always thought dark matter was a load of hot gas, but now it seems it's not even that! QI for the win.
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References
Liu et al., 2025. Discovery of a high-velocity cloud of the Milky Way as a potential dark galaxy. Science Advances, Vol 11, Issue 16. https://arxiv.org/pdf/2504.09419
McCulloch, M.E., 2012. Testing quantised inertia on galactic scales. Astrophysics & Space Sci. Vol. 342, No. 2, 575-578. https://arxiv.org/abs/1207.7007
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