Astronomers have announced that they have found two galaxies without dark matter.
Is this a big deal? An observational anomaly?
Something else?
I’d say whatever else it is, it’s another nail in the coffin on MOND or similar “there isn’t really dark matter – we just don’t understand gravity” theories. If there are galaxies that don’t have the gravitational effects associated with dark matter, then the effect must not be intrinsic to how gravity works.
What is MOND?
Modified Newtonian Dynamics. Actual explanation of any of this is way above my pay grade. 
Modified Newtonian Dynamics. It’s a just-barely-above-crackpot model that’s pretty much pushed by only one physicist, claiming that dark matter can be explained by different gravitational behavior. But there are so many things that it breaks, that the model ends up being composed mostly of duct tape.
But that coffin was already pretty securely nailed shut. What I think is most interesting is that we have no idea how such a galaxy would form, either how the dark matter could have been excluded in the first place, nor how the baryonic matter could coalesce without the dark matter.
I’m not saying that these galaxies are home to Type III civilizations, or anything. But I’m also not saying that they’re not home to Type III civilizations.
That, or it’s some sort of observational error, and they have dark matter just like everything else. There will obviously be a lot of work to verify (or refute) this from other astronomers.
It’s a pretty big deal from what I understand as the current thinking is that dark matter is required to form large galaxies. I don’t know what the models say about these two possible galaxies without it, but my guess is some folks are freaking at this point.
I would guess (and this would be the WAG of all WAGs) that it’s some sort of observational issue, and that later or better study will find something that explains it. But dark matter is one of those things that are still completely beyond even the smartest and most studied on the subject…and I’m not even in that same universe as those guys and girls. Maybe one of the boards astro-physics types will be along to explain better (hopefully using simple words…maybe a stick drawing).
It’s aliens!!! It’s aliens!!! Chronos said it’s aliens!!! Or observational error. Or it’s something we just don’t understand yet. Or it’s aliens!!!
"In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves. But Chronos said it’s aliens!!!" – (mostly) Carl Sagan
And hundreds of thousands of internet commentators who really don’t like dark matter. 
Just to rule that out, someone should observe them in the IR. A type III civ should be very bright in that range. However, note that these two galaxies have a number of very bright globular clusters. The civilizations clearly haven’t extended to those yet.
There’s a somewhat more technical article, but still accessible to the interested layman, at Sky&Telescope. The first of these galaxies was actually reported about a year ago; the other just this year. They’re both in the huge Coma Cluster, on opposite sides (but assumed to be related to) a large elliptical galaxy, NGC 1052.
Another thing is they are (probably) members of a type of galaxy called Ultra-Diffuse Galaxies. But most UDGs have lots of dark matter. There’s one suggestion that UDGs are actually just oversized dwarf galaxies; that is, they have about the same amount of matter as a dwarf, but they’re spread out to large galaxy size.
Well, that’s giving it a bit short shrift, really. There’s lots of researchers working on various versions of modified gravity including MOND, and there are different models fitting various sets of observations more or less well—some better, some worse than dark matter. At any rate, the observation of galaxies lacking the dynamics usually attributed to dark matter isn’t in and of itself a problem for MOND and allied programs—modified gravity adds additional fields to the gravity side of the Lagrangian (just as dark matter adds additional fields to the matter side), and their effects are going to depend on their initial values and the system as a whole. Anyway, for a view on how these observations do not falsify modified gravity, see e. g. Sabine Hossenfelder’s blog (more resources therein).
As I understand it, it was explicitly developed as a ‘How can we explain the orbits of stars without dark matter, based purely on Newton’ theory and took off from there.
And it’s a good thing that people are looking into alternatives to dark matter. We have yet, after all, to find any dark matter.
We have found dark matter. We’re made of dark matter, after all.
Unless you mean non-baryonic dark matter, which is most of it. But we’ve found some of that, too.
Which still leaves most of the dark matter unaccounted-for. But really, it would be surprising if there were not plenty of particles that didn’t couple to the electromagnetic force. Dark matter, not its lack, should be the default assumption.
All that said, yes, it’s good for people to look into alternative models, because that’s how science works. And when those attempts at alternate models consistently fail and have to be modified to ever-increasing levels of complexity to take into account every new observation, while the simple models continue to be consistent with every new observation with only slight if any modifications, that’s when science says to just use the simple model.
That’s not correct. We haven’t found anything; we just expect something to be there. Dark matter makes the numbers work in current theories (in much the same way that MOND fiddles the numbers to make galaxies work) but actual evidence of dark matter is missing. We haven’t identified it or anything. Dark matter is currently entirely hypothetical. It’s the #1 hypothesis, but it’s still a hypothesis.
Just as Einstein refined Newton, so someone else may refine Einstein. Who knows? There are many scientists working on it, and if they do the ramifications of their efforts will shake the world. Equally, if a scientist produces a bottle (or whatever) of actual dark matter then that too will shake the world.
We live in exciting times.
Physicist fight! Watch out for flying slide rules! 
When? Who? I’ve never heard this.
Neutrinos are non-baryonic dark matter.
I’m sure it means something. What that “something” is I don’t know.
MOND and MOND. What is MOND?
As far as I know we’ve never directly detected nor even know what dark matter is (it’s why they tack ‘dark’ on it…that, and it just sounds cool). All we have is indirect evidence (like how light bends around…something…or how galaxies don’t have enough detectable mass to be able to hold together, or how the outer stars in a galaxy aren’t traveling at the speeds we estimate they should if the only mass was what we can directly observe).
I think what some posters are saying is that neutrinos don’t account for all non-baryonic dark matter.