New studies confirm existence of galaxies with almost no dark matter

Thinking more on this... Those are either 90% or 95% confidence intervals (couldn't tell from the recent paper).

So that would mean if you looked at 100 galaxies you would expect 10 (or 5) to fall outside the range if the model was correct.

> Am I wrong to conclude that this result itself is an argument for the existence of dark matter?

Only if competing arguments that account for the rotational speed discrepancy in galaxies fail to explain "galaxies without dark matter", which... I think they do? (I am not 100% sure on this).

Of course occam's razor is only a heuristic, and not a law, but we would prefer a theory that is simpler, and if we have to add an extra level of indirection explaining why some galaxies lack dark matter, then that is less desirable in a theory. Repeating myself, science doesn't necessarily do what we desire.

I think that is a reasonable conclusion to make (assuming these findings are correct). The article says the same:

>Ironically, the lack of dark matter in these UDGs strengthens the case for dark matter, the researchers say. It proves that dark matter is a substance that is not coupled with normal matter, since they can be found separately.

I've been very reluctant to go 100% with the Dark Matter theories, preferring to think that Kepler's Law was somehow the root problem. Note, I had no real basis for these thoughts!

These findings do indicate that the Dark Matter theory is increasingly reliable, in that some galaxies are shown to have different densities (& thereby there's nothing wrong with Kepler's Law, and further proof that my "going with the gut" when compared with empirical science is just silly!)

These findings help reinforce that Kepler's Law are working well and seem to indicate that that dark matter is not necessarily tied with matter, but is hanging around in different densities in different galaxies.

> Ironically, the lack of dark matter in these UDGs strengthens the case for dark matter, the researchers say. It proves that dark matter is a substance that is not coupled with normal matter, since they can be found separately.

To me, it's a bit like how our planet is not uniformly the "same stuff", but we have deposits of Gold and Coal and Granite spread around the place in an uneven manner.

It is known that certain types of galaxies can have less dark matter than others. I believe that jury is still out. I'm a little concerned by how many people "believe" there is dark matter. This adds a bias to the proposed solutions and the funding for each. It leads me to be a little skeptical when I hear that dark matter is being found and whatnot through the speed of stars. I'd love for research to be more balanced since the phenomena can be explained through more hypothesise than just increased mass.

Both galaxies without dark matter are UDG's.

I'm not a physicist, but this strengthens my personal hunch that dark matter doesn't exist. My heretic theory is that it is just a product of some sort of physical law acting as a "ceiling" in local gravitational force which gets conserved by spreading over large distances.

They are called "Dark Galaxies", there are a few candidates, but IIUC they are not confirmed. https://en.wikipedia.org/wiki/Dark_galaxy

https://en.m.wikipedia.org/wiki/Dragonfly_44 might be?

If you could play God and place blobs of dark matter in the right places and moving at the right speeds, it seems plausible that such a dark matter Galaxy could physically exist and be stable.

But I don't think we know of a mechanism for such a structure to form naturally. Normal particles collide and transform some of their gravitational energy into heat. If dark matter only interacts gravitationally, blobs of it would pass right through each other without losing energy. I think(?) you need these sorts of collisional energy losses to get matter to coalesce into a galaxy.

As another commenter has noted, they wouldn't then be galaxies. And the question of whether there are (or how common and at what mass) galaxyless dark matter overdensities (think of it as a clump, in the literature they are called "halos") is still open.

One of the problems with the current best model of the universe (Lambda CDM - a universe that is primarily make up of some cosmological constant driving expansion and cold dark matter whose gravity drives structure formation) is that it predicts that there should be far more of these dark matter overdensities than we see galaxies. This is the "Too big to fail" problem - https://arxiv.org/abs/1103.0007 is a pretty readable description. Basically the model predicts lots of dark matter clumps orbiting the milky way and we don't see enough satellite galaxies to populate them.

So there are now two options: 1) Galaxy formation becomes stochastic at lower dark matter halo masses - and so there are lots of these galaxyless overdensities of dark matter. 2) There is something wrong with our model and there might not be these overdensities. 3) The milky way is an outlier.

I think the short summary is that, at very low masses (say ~1e6 solar masses) we wouldn't be surprised if not all dark matter halos form galaxies. At high masses (1e12 solar masses, roughly that of the milky way halo) we would be very surprised if there weren't galaxies. But in between we are not sure.

Edit: To be clear, these structures would still likely contain baryons (normal matter), they just haven't formed stars (or would have formed only a very few stars).

I think the main theory for how the dark matter free galaxies form is that they somehow lost their dark matter component. Perhaps in a collision with another galaxy.

So in that scenario it's quite possible that the dark matter part of those galaxies is drifting around on it's own somewhere. I don't know that we'll have any way of detecting them soon though...

How does dark matter interact with another dark matter? I know that dark matter can only interact with normal matter by gravitational force..

It surely wouldn't be a galaxy then? I think a galaxy has to have stars in order to be a galaxy.

> because of dark matter's lack of electromagnetic interaction

This is still an assumption, because we don't know for sure that dark matter is only weakly interacting, but this is what was observed in the Bullet Cluster collision: https://en.wikipedia.org/wiki/Bullet_Cluster

There's a good explanation here: https://astrobites.org/2016/11/04/the-bullet-cluster-a-smoki...

The lensing was found to be strongest near the visible matter (stars), and the amount of observed lensing couldn't be accounted for with the total stellar mass alone. So the assumption is that the dark matter, like the stars, wasn't slowed much by the collision. The gas, on the other hand, interacts electromagnetically and slows down in the middle.

That's not to say that the stuff that stars are made up of isn't affected by electromagnetism, but it's a short-range effect and even in a collision, interstellar separation is large (i.e. contrary to what you might think, a galaxy collision doesn't involve all the stars smashing into each other, most of them will just sail right on through). Gravity will dominate in that case.

The most famous example: the Bullet Cluster

https://www.forbes.com/sites/startswithabang/2017/11/09/the-...

“This argument has been made for over a decade, now, with no satisfactory counterargument coming from detractors of dark matter. It isn’t the displacement of gravitation from normal matter that “proves” dark matter exists, but rather the fact that the displacement only occurs in environments where dark matter and normal matter would be separated by astrophysical processes.”

When galaxies "collide" stars barely ever get into contact. The interaction is completely dominated by gravity.

Why should it stay there? Imagine you had a sphere of particles with an initial velocity of 0 that interact only through gravity. At first, these particles would all accelerate towards the center of gravity. Once they reach the center of gravity, they would have accumulated some velocity, and so continue to travel away from the center of gravity (but still be accelerating towards it). Eventually, the acceleration towards the center of gravity would 0 out their velocity again, and they would start falling back towards it. The end result is that your original sphere oscillates indefinantly, but always returns to its original state.

As your initial configuration becomes more random, this discernible time structure begins to fade, and you end up with a voluminous cloud.

What allows regular matter to collapse into dense structures is that it interacts with itself to shed velocity.

I think it's like this: Imagine you have a cloud of dark matter, with a combined centre of gravity. Now imagine a "piece" of dark matter falling down to that centre of gravity. It doesn't impact, or slow down with friction, or anything like that, it just keeps going, through the centre, and out the other side, just as far as it fell before. There's no way to clump together, or stay in the centre.

But it's not electromagnetism etc. which stops the ordinary stars from falling into the middle of the galaxy, either. It's just gravity, they are slowly orbiting the centre of mass of the galaxy.

Maybe it has some sort of internal pressure. Just like our planet doesn't simply collapse into a black hole.

The fact that it doesn’t interact should make it less likely to fall in as it still has intertia (apparently).

The existence of dark matter in galactic halos is inferred from the velocity dispersion of stars in a galaxy (sigma). It’s important to realize that this is a very rough kind of estimate because every aspect of it (as in the case of the M-sigma relation) is roughly estimated. Virial mass is inferred from average temperatures of gas by way of detexting X-rays. Central black hole mass is inferred through the M-sigma relation, and stellar masses are inferred through luminosity. Every so often a new paper comes out revising the overall estimates, but the proportion of DM to luminous matter stays pretty constant.

Now, in a sense this makes a “selfie” easier, because you just need a series of snapshots which help you make all of these inferences. On the other hand because we’re in the disk of our galaxy it is difficult to image because we have to deal with all of that hot interstellar gas and dust, and the extremely bright galactic core. I’m not sure how much, when you take into account a long period of time and many wavelengths can be observed, but I’d say we can observe the majority of our galaxy in at least one wavelength. Still, we’re not talking about “selfies” here, just really well educated guesses based on observations of our own galaxy and similar spiral galaxies.

So as far as inferring the existence of a dark matter halo in our own galaxy we’re pretty certain, and it seems to make up about 95% of the galactic mass. It’s based on something a lot more tenuous and complex than a “selfie” though. Once you’ve done your best to estimate the total luminous mass of the galaxy and it’s rough distribution though, and ruled out MACHOs with lending surveys all that’s left in the remaining narrow window is dark matter. The only other theory with any chance at all is MOND, but it has to thread crushingly narrow passages between observation and tests of GR.

Isn't it to do with the rotational velocity? If it isn't what you'd expect from the matter that you see, you need DM for it to add up.

Also from my armchair view as well.

Both of these galaxies are UDG's, keep in mind.

In my uneducated opinion dark matter is an outcome from cluttering too much gravitational force in too small a space.

One alternative is 'Process Physics' which is not widely accepted but has a fundamentally different explanation for dark matter: Turbulence in the in-flow of space into matter. This flow is PP's model for gravity itself, as if matter continuously 'consumes' space to continue its existence. It's an interesting, quite different and generally totally rejected point of view.

i dont understand what you are saying. What would accumulate?

At least I can say, everything propagates across time. No time, no propagation.

Pretty much all massive objects except black holes emit light/radiation, and black holes, afaik, are detectable. Thus there's something else that we give a placeholder name "dark matter". It's possible it's not a matter of any kind at all, but just some very smooth but irregular field that interacts with the space-time curvature (gravity).