Still don't fucking get it lol. Between this, AI, and even just the drumset, I wish there was more time in the world to learn all the things I wanna learn :(
Try this: when we look at a galaxy, we can see how fast it's rotating. We can do this using spectrometers that can see where light from stars is red-shifted (which indicates those stars are away from us) or blue-shifted (moving toward us).
The faster a galaxy is rotating, the more massive it must be, because without that gravity the stars would just be flung out into space like toddlers on a malfunctioning carousel.
We can also make a pretty good guess at the amount of "normal" matter in a galaxy (i.e., not dark matter) by measuring how bright a galaxy is, since nearly all light comes from stars, which are, as far as we know, composed entirely of normal matter.
So, putting those two things together, we encounter a mysterious thing: having observed thousands of galaxies, nearly all are rotating much faster than is expected given their estimated mass. One possible explanation for this is that there's another kind of matter in there that doesn't emit any light, which we call dark matter. To be more precise, not only can this dark matter not emit radiation, it cannot interact with radiation at all -- it can't absorb or reflect it. Otherwise we'd see it, just as we see clouds of dust in galaxies (which is accounted for in our mass estimates).
Other proposed explanations involve changing the way gravity works. These are very interesting and compelling ideas, however this newly discovered galaxy would appear to weaken the case for modified gravity theories, because under those theories we would expect this galaxy to be rotating as fast as all other galaxies, since gravity is a function of mass. But that's not what we see; this new galaxy is rotating much slower. It is, in fact, rotating at the rate we would predict if it didn't contain any mysterious invisible "dark" matter. Which is why we think it doesn't!
So if whatever a bunch of stars were orbiting weren't massive enough (all the other stars in the galaxy), they wouldn't really even be orbiting, huh? That's what necessitates dark matter - there doesn't appear to be enough matter to justify the amount of mass it would take for all the stars in a given galaxy to actually be in a galaxy style orbit?
Now, one possible alternative to dark matter is that gravity itself doesn't behave the way we think it does. There is a group of theories that we generally call MOND (MOdified Newtonian Mechanics), which propose that gravity is stronger than we think at very low levels of acceleration. When is acceleration very low? Well, it's low when an object is very far away from any other sources of gravity, and this actually describes most stars in galaxies. If a star isn't too near the center of a galaxy, the gravitational force on it is extremely small -- much smaller, for example, than the pull of the sun on the earth. Most stars are traveling in nearly a straight line, bending only ever so slightly on an enormous circular path. That bending is caused by a teeny tiny acceleration -- a gravitational pull -- toward the center of the galaxy's mass. Which is always more or less right in the middle.
MOND theories propose that as objects get farther away from each other and experience less gravity, the gravity still remains stronger than is predicted by classical newtonian mechanics. Thus, the gravity experienced by most stars in a galaxy is greater than we think, and therefore they must rotate faster for the galaxy to maintain a stable shape.
These are attractive theories IMO, and they explain some things that dark matter has difficulty explaining. However, the galaxy described in the article is a serious problem for MOND, because MOND says that it should be rotating as fast as all other galaxies of its size. But it isn't. It is rotating at the rate predicted by the classic newtonian model of gravity, which would naturally seem to rule out any other modified model of gravity.
If you look at a galaxy, you can measure the amount of visible mass using star luminosity, and you can calculate the total mass of the galaxy under standard cosmology using orbits, radial velocities and so forth. Comparing the visible mass to the calculated mass reveals a huge discrepancy. Some of the undetected matter can be normal, baryonic matter, but there is to much missing matter for it all to be normal matter. So we postulate the existence of an unknown type of matter that interacts gravitationally but not by radiation (light).
Using MOND, when you calculate the the mass of a galaxy and compare it to the visible mass, you still get a discrepancy! This discrepancy is much smaller however. Under standard cosmology it's a factor of 10-100 times, under MOND, the mass discrepancy is usually in the neighborhood of 2. In other words, under MOND, there's still matter in the universe that we can't detect by its light, but the missing mass is small enough that it can be explained by normal matter that's to dim to detect. This is why things like the bullet cluster are actually expected under MOND.
I don't know the particulars of this exact galaxy, but it's possible that the radial velocities that astronomers measured for its stars have a high enough margin of error such that it still fits within MOND's predictions. Something I've noticed is that MOND is still fringe enough that you can't necessarily trust non-MOND physicists when they talk about it. One thing is for sure however, this galaxy is as puzzling for Dark Matter as it is for MOND.
"Intuitive" means two different things in this context. One is that, over time, theoreticians certainly gain a lot of intuition about how the relevant math works. That's no substitute for doing the work, but it does cut down on the number of mistakes you make, you often have a decent sense of what's definitely not going to work, etc.
The other meaning is what people usually imply - that some combination of analogies with everyday phenomena finally "clicks" and you "understand" the physics all of a sudden. That's just nonsense. Past the natural limits of our everyday intuitions (which are very narrow indeed, consider how something as trivial as terminal velocity is often unintuitive to students in basic mechanics) physics is applied math, everything else is a crude approximation at best.
I will say that what can become intuitive to us (in your second sense) can certainly change over time, at least after generations. Netwton’s first law must not have been intuitive, mostly because of the ubiquity of friction in everyday life, but I think it is fairly intuitive now for people with a solid basic science education. Maybe more advanced concepts will get more intuitive over the generations, as science concepts get taught at younger ages.
They click about as much as a well designed but strange piece of code suddenly start to make sense.
Also feeling dumb is part of the process .