Main HN discussion is here:

https://news.ycombinator.com/item?id=12702759

Incidentally, it is interesting to see how many folks uniformly have the same misconception that dark matter is only about explaining galactic rotation curves. Obviously it's not at all surprising or troubling that laymen aren't experts, but the mistaken information is really far from random.

To answer questions:

1) We still need dark matter. This mostly focuses on small satellite galaxies. Not galactic rotation, galaxy cluster interaction, or light lending though Galaxy Clusters.

2) The main interesting tidbit is that it seems the density of Galaxies in the Universe has changed sharply in the past 1billion years. Which may lend more data points to Dark Energy/Inflation models.

Could that mean that we don't need a dark matter theory?

I don't think so. Dark matter is needed to explain the contents / behavior of galaxies we can see. This article is talking about galaxies we can't see.

On that note, here's some interesting / related recent news in terms of a galaxy we have a hard time seeing: https://www.washingtonpost.com/news/speaking-of-science/wp/2...

It's very important to note that unlike what the media reports, there is no new classification in astronomy of "dark matter galaxies" added due to this discovery.

This is the original paper for Dragonfly 44 https://arxiv.org/abs/1606.06291

There are a lot of question about the composition of that galaxy, and what makes it so dimm, but we need a lot more observations of that galaxy in order to even start asking the right questions including full spectral analysis from multiple observations and tools, correct redshifting and depth placement, ensuring that there is nothing in the way that could explain why it is so dim and many more other factors.

We also need to find other candidates of highly/ultra diffused galaxies in order to even start to try to figure out what is going on.

Yes I read the paper and I know what you mean, the media loves to jump to conclusions. Of course, they're also the reason I even found out about it. I'm super excited to read what comes from the future observations.

No. Dark matter is needed to explain the rotation curves of individual galaxies. That there were more small galaxies in the early universe doesn't help with this.

Potentially, there likely still be "dark matter" but it might we might not need e a new classification of dark matter.

We've known for a while now that there seems to be more galaxies out there, heck before the hubble went up we didn't even knew how much there is.

There are also indications that there is considerably more intergalactic material out there than we've suspected before.

Howeveer the problem is that "dark matter" is also needed on local scales, including our solar system; this is a good read http://scienceblogs.com/startswithabang/2013/01/18/why-the-u...

So "dark matter" is needed to explain our current model of gravity both local and global scales; it is important to note tho that "dark matter" is a placeholder it doesn't necessarily have to be a new type of matter (e.g. AXIONs), some irregular mode for normal matter (e.g. WIMPs) or or it can be regular matter which isn't easily observable (e.g. MACHOs) or it can be also be any combination of the above, and things we haven't even considered yet.

Alternatively we might need to rethink our understanding of gravity, and of the existing forces in general, a good point to note is that gravity isn't a traditional force in the first place which already sets it apart from the bunch (it's a property of the distortion of spacetime, it also is only attractive unless dark energy is somehow a very low/zero value property of the gravitational field).

So gravity might not work like we think it does, it might be hiding another elementary force, or it might be that gravity on certain scales/conditions splits into multiple forces similarly to how the electroweak force works (at some point during/prior to the big bang all forces/fields were unified at least in theory).

Mainstream Physics have been stuck somewhat for a fairly long time on certain aspects, there is also quite a bit of misconception about what works and what doesn't for example most people think that quantum gravity doesn't work which it does, you can use it for everyday life and for many other things, you can't use it during the big bang (and shortly after) and it breaks down near very strong fields e.g. a singularity which is also the internal killswitch or "mathematical contradiction" embedded within general relativity where gravity and spacetime as we know it break down.

> Howeveer the problem is that "dark matter" is also needed on local scales,

The link you provided doesn't seem to say that:

"Well, since that time, there have been additional challenges to how gravitation works in our Universe, albeit on much larger scales than our Solar System.

From the 1930s to the 1970s, galaxy clusters and individual galaxies had their speeds measured very precisely for the first time. This meant that the speeds of individual galaxies in clusters could be measured relative to the center-of-mass of the cluster itself, and the rotational speeds of spiral galaxies could be measured relative to the center of the galaxy itself. In both cases, it was found that the motions did not line up with the predictions of general relativity in a Universe where matter was made up primarily of protons, neutrons, and electrons."

It does if you are slightly more familiar with the subject, it's needed to account for the density and the mass of interplanetary medium as well as quite a few other things like the flyby effect, this changes with the more we learn about the outer solar system and our closest stellar neighbours.

That said overall I'm in the camp of gravity is incomplete and dark matter is likely to be normal matter we can't and therefor don't yet account for.