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Galaxy Without Any Dark Matter Baffles Astronomers (arstechnica.com)
A distant galaxy that appears completely devoid of dark matter has baffled astronomers and deepened the mystery of the universe's most elusive substance. The Guardian reports: The absence of dark matter from a small patch of sky might appear to be a non-problem, given that astronomers have never directly observed dark matter anywhere. However, most current theories of the universe suggest that everywhere that ordinary matter is found, dark matter ought to be lurking too, making the newly observed galaxy an odd exception. Dark matter's existence is inferred from its gravitational influence on visible objects, which suggests it dominates over ordinary matter by a ratio of 5:1. Some of the clearest evidence comes from tracking stars in the outer regions of galaxies, which consistently appear to be orbiting faster than their escape velocity, the threshold speed at which they ought to break free of the gravitational binds holding them in place and slingshot into space. This suggests there is unseen, but substantial, mass holding stars in orbit. In the Milky Way there is about 30 times more dark matter than normal matter. The latest observations focused on an ultra-diffuse galaxy -- ghostly galaxies that are large but have hardly any stars -- called NGC 1052-DF2. The team tracked the motions of 10 bright star clusters and found that they were traveling way below the velocities expected. The velocities gave an upper estimate for the galactic mass of 400 times lower than expected. The researchers described their discovery in the journal Nature.
There are enough unrelated discrepancies in our observations, that can all be explained by dark matter or dark energy (making up a consistent fraction of the universe in all cases), that I think it's safe to say there's probably something to it. Especially when we know the Standard Model in particle physics is flawed, and many extensions happen to contain viable dark matter candidates.
For dark matter specifically, I would say the smoking gun is the aptly named Bullet Cluster. Two galaxies colliding, ordinary matter interacts and gets slowed, dark matter doesn't interact and gets separated from the ordinary matter, and then the dark matter and ordinary matter components can be observed separately (ordinary matter from ordinary photons, dark matter from its gravitational lensing effect on photons coming from galaxies behind the cluster).
Except rotation speeds have already been explained ... in other ways.
But you don't just have to explain rotation curves: you also need to explain the velocity dispersion of elliptical galaxies, mass estimates of galaxies clusters, gravitational lensing patterns, etc. All told, there are at least eleven pieces of evidence for dark matter, of which the galaxy in this story is one: if our theories of gravity, for example, are wrong, and rotation curves should actually look as they do, then why is this galaxy an exception?
"dark matter/energy" is merely a convenient identifier for the discrepancy and is merely saying "we don't know yet"
This is completely correct! But we do know a few things. We know that dark matter must interact with itself a bit, to be able to form small halos - but not too much, so it doesn't condense into black holes. It must move substantially slower than the speed of light (which rules out neutrinos as dark matter) or else the large-scale structure of galaxy filaments, which it seeds, would be more spread out and less lumpy. We know that it must have about five times as much mass as the regular matter in the universe.
The best term I've seen for this is "parameterised ignorance". We know there's *something* there, and we're gradually setting limits on the properties it could have. If a hypothesis predicts a particular set of parameters, and we exclude them, then we throw away that hypothesis. There's still a lot of parameter space left, including a load of hypotheses (plus whatever we haven't thought of) ... but every experiment that chips away at this space brings us closer to pinning it down.
Hereis a non paywalled source. This is interesting because it is yet another data point that our understanding of gravitational forces on normal baryonic matter (regular stuff) can hold at at least a few kilo parsecs.
While it's true we don't know if dark matter is an effect, or an actual stuff like wimps (weakly interacting massive particles), the evidence of uniform dispersion from gravitational lensing to kinematics while the uniformity still clumps in a gravitational way is starting to be convincing. A great example is the bullet cluster where the friction of the colliding gasses slowed the normal matter but the dark matter was virtually unaffected, the gravitational attraction was too weak and the dark matter stripped from the baryonic matter. But further, computational models consistent with measurement rely on dark matter to have a cooling effect which wouldn't work (or it's not at all clear anyhow) if it wasn't particle like. If I was a betting man, I'd put my money on a weakly interacting particle.
This suggests there is unseen, but substantial, mass holding stars in orbit. In the Milky Way there is about 30 times more dark matter than normal matter.
This is an improper statement of what we actually know. It's like saying a UFO must be an alien from another planet while forgetting what the U stands for. We have close to NO IDEA what the phenomena we call dark matter actually is so saying there is 30X as much of it is a nonsensical statement. It could be some sort of matter but we are not at all certain of that. You could say that our current models of gravitation due to matter only explain a few percent of what we see and that would be an accurate statement of what we know. It's possible that the 30X statement is correct but we don't know that yet. If "dark matter" ultimately turns out to be some flaw in the model of general relativity or the like then saying there is 30X as much dark matter as "normal" matter will sound idiotic. If we want to talk in terms of force then fine - saying there is 30X as much gravitational force makes perfect sense.
Short version. We don't know what it is so it's illogical to keep saying how much of it there is until we know what it is.
$1.7 billion, to be precise. And we didn't even get all of our hostages back:
http://www.latimes.com/nation/...