Quick search & ran across a recent talk (April 2016), which included this–

“Ultra-diffuse Galaxies: the Low Surface Brightness Tail of the Abundant Dwarf Galaxy Population”
Nicola Amorisco
(15 minutes)
and–

“Distribution of Dark Matter versus Luminous Matter”
Rennan Barkana
(15 minutes)
both contained within the video here:
https://youtu.be/r-9-Wwflq_k

>>Very technical stuff and I haven’t had a chance to watch them multiple times (and get a grip on the content) but very interesting.

However, for a spherical body with uniform density, gravity decreases as we travel deeper into it, such as in a mine. If we had a mine all the way to the center of the Earth (or the Moon, to avoid Earth’s magma and other nasty interior attributes), we would find that gravity decreases linearly with depth, until it reached zero at the center. This is because as we go deeper, there is more mass above us than when we are on the surface, and that mass pulls upwards, not downwards.

Issac Newton was once asked what shape orbits would take if gravity decreased with the square of distance, and he replied immediately that they would be elliptical, because he had already done the math. Imagine what orbits would be like if they could exist inside the Earth, with gravity increasing linearly with distance.

A galaxy has similar gravitational properties, as there are stars farther out than our sun, pulling on our solar system, as well as stars closer to the Milky Way’s center. This all affects the orbit of our solar system (and all stars) around the galaxy’s center. Since the galaxy is not uniformly dense, individual stars affect the motions of other individual stars and their solar systems, but the overall concept is similar to my inside-the-Earth thought.

Loads of fun, that orbital mechanics.

Astronomers have found that galaxies generally do not spin the way early models predicted (thus the models were wrong), and to make the models match reality, they have to add mass that is not seen in real galaxies. If this mass were dust and gasses, then they would glow, in infrared, and reflect light in ways that are not observed in the galaxies, so Astronomers have concluded that there is something other than dust in galaxies. Unless gravity does not work the way we understand it, something is causing — or more accurately: something is being — more mass distributed throughout galaxies than we are now able to observe.

What a mystery!

Garry: This is an understandable error, compounded by astronomers who have named their other big mystery about the universe’s expansion rate as “dark energy.” Both deal with the total mass of the universe, but deal with significantly different questions.

They do however speak directly to what Mr. Z noted above, “…all the data they have gathered of that accelerated motion in the last half century consistently points to a cloud of unseen mass that all galaxies appear to sit within.”

“Cosmology for Particle Physicist’s”
Part II – Introduction to Cosmology
Dr. Edmund Bertschinger
https://youtu.be/AKQF5MMWOQM
This is highly understandable, although has some heavy math. (which I just take as given & concentrate on the implications thereof) It’s part of a multi-lecture class aimed at informing Particle Physicist’s of Cosmological Theories.
(The ultra-small is intrinsically wrapped up with the ultra-large.)

“Demystifying the Higgs Boson”
Dr. Leonard Susskind
https://youtu.be/JqNg819PiZY

Heavy math as well, (lots of Field Theory) but totally within the realm of a large portion of BtB readers. (I just assume that if I can comprehend a bit of this, it’s infinitely easier for folks who are more technically educated than myself.)

I have stayed at numerous Holiday Inn Express’s but I only play a Physicist/ Cosmologist, on the interweb.

Garry wrote: “My first thought upon reading this post was that maybe this is the alternative explanation to dark matter.”

Nope, these dim galaxies don’t do anything to explain dark matter. It is important to understand the source of the mystery behind dark matter to then understand why it is really a puzzle. Numerous studies of many many galaxies and the movement of the stars within them has consistently found that the stars move much faster in the galaxy’s outer regions than they should, as if there is far more mass out there on the periphery than we can see..

The easiest way to explain this is to use our solar system. As you move out from the Sun, the orbits of the planets get longer and longer, both because they have farther to travel to complete their circle but also because they are simply moving slower. There is less matter out there to pull on them. However, if the solar system was like most galaxies, the outer planets would not orbit slower, but as fast if not faster than the inner planets.

Thus, something is causing the outer stars in galaxies to move faster. Scientists have no idea what that is, but all the data they have gathered of that accelerated motion in the last half century consistently points to a cloud of unseen mass that all galaxies appear to sit within.

These new dim galaxies are not in a position to explain this dark matter mystery. In fact, given time, I am willing to bet that astronomers will find that their outer stars also move faster than they should, given the visible mass that we detect.

Total tangent– I have to ask– do you translate any Japanese Anime? (Totally NOT my thing personally.) You could wind up with a 4th job!

It’s human nature to try to find patterns, and over the ages science has been too ready to fill in the blanks without any real evidence, or worse yet, to doctor data to make it look like something predicted by the theory is really happening (see change, climate).

“I have no doubt that in reality the future will be vastly more surprising than anything I can imagine. Now, my own suspicion is that the Universe is not only queerer than we suppose, but queerer than we can suppose.”
J. B. S. Haldane

I would take this opportunity to shill for Gresham College Astronomy lectures, and in particular this recent talk–

“The Dark Side of the Universe”
Prof Joseph Silk
http://www.gresham.ac.uk/lectures-and-events/the-dark-side-of-the-universe
(April 2016)

>Wherein he discusses Fritz Zwicky, the Coma Cluster, Dark Matter & Dark Energy.