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Comment by fhars | Hacker News Reader

A more damning quote seems to be Another recent probe found galaxies inconsistent with the theory of dark matter, which posits this hypothetical substance to be everywhere. There is no reference given, but I have the suspicion that it refers to the discovery of galaxies with almost no dark matter (cf. https://news.ycombinator.com/item?id=16701248). That does indeed refute the assumpion that dark matter is everywhere, but that is not an assumption anyone makes and is not part of any relevant theory of dark matter. On the contrary, the existence of galaxies like that is generally seen as one of the strongest arguments for the existence of dark matter as an independent, if elusive physical entity.

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Couldn't it just as easily mean that our predictions based on general relativity are wrong?

Either 1) the prediction for a particular galaxy is right, and there's no "dark energy" or "dark matter" or

2) the prediction is wrong, and you add a bunch of dark energy or dark matter to make the equations work out.

It's not testable at all - no matter whether predictions for a particular galaxy are right or wrong, you're still allowing yourself to think that the theory is correct. How is this scientific?

It's not that simple. It's not like a) Some galaxies fit predictions, b) Some do not, and we add unknown dark matter.

It's that when they do not fit predictions, there is a single parameter (the mass of dark matter) that does explain its behavior, whose model is that of a weakly interacting gas halo. So dark matter tries to fix things in a very precise way, it's not an arbitrary curve fitting exercise.

Now whether there is really weakly interacting matter out there or some other explanation is valid is still an open question afaik; but the evidence is quite strong so far.

> Now whether there is really weakly interacting matter out there

Not "out there". It's supposed to be >80% of the matter of the universe. We supposedly are eating some every day.

> It's supposed to be >80% of the matter of the universe. We supposedly are eating some every day.

It's >80% of the average density of the matter in the universe. But that's an average over all of the universe. On the scale of our planet, or of the solar system, that density is negligible--as in "30 orders of magnitude smaller than the density of the stuff you eat" negligible.

No, the only consistent explanation for both the overwhelming majority of galaxies as well as the few outliers is general relativity and independent dark mater. All other theories that make testable predictions (modified newtonian dynamics or MOND seems to be the only class that is not completely add odds with almost all observations) cannot explain the behaviour of those galacies that are as big as other galaxies but rotate wih a different speed. But the difference is easily explained with different amounts of dark matter in the galaxies, and the bullet cluster cited elsewhere in this thread give a plausible explanation of how these galaxies might form.

> Couldn't it just as easily mean that our predictions based on general relativity are wrong?

No, because general relativity with dark matter (and dark energy--the best current cosmological model we have) predicts a lot more than just galaxy rotation curves. And the whole point of the model is to explain all the things it makes predictions about, not just one.

Not necessarily. This great video from PBS Spacetime is a fairly detailed, but good for a 'nearly' layman on the findings in depth. https://www.youtube.com/watch?v=5t0jaE--l0Y

A discussion on how a galaxy could be missing dark matter - or how it could be separated from a galaxy

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