Drums vibrate because they are in tension. That is there is a restoring force. But I would think that massive bodies and Dark Matter, once deflected gravitationally by a smaller galaxy or two, would just coast and continue separating over time. If our galaxy is indeed vibrating like a drum, then Dark Matter must be subject to a greater force than gravity. Perhaps the UCB astronomers are just being poetical
Matter = disturbance
"What is the matter?" as in "what is disturbing you?"
Dark matter = dark disturbance = real alternatives = real energy = conserved energy.
Gravity = coming together.
Dark matter = conserved alternatives = at least 3 dimensions (the two alternatives and the conservation) = a freedom surface greater than coming together (as gravity force = just 3 dimensions as it is "coming together" so requires just two items that come together freely i.e. via a third that they negotiate with).
Dark matter IS a force greater than gravity (ta yanniru for that link!)
PARALLAX ERROR and DARK MATTER:
It was once thought that the observed lack of relative change of position among the background stars regarding each other
(a concept that is the same idea as modified Newtonian dynamics as: Newtonian dynamics = action at a distance (dynamic = action; Newtonian = at a distance (or simple relativity); "modified Newtonian dynamics" = modified action at a distance = imaginary distance = "observer error" i.e. parallax error).
meant that the Earth did not go around the sun.
This is cited as an example of Karl Popper's "falsificationism" failing; as observation appeared to falsify the proposition that the Earth moved around the sun. BUT 300 years later or so it was found that the background stars were much further away than previously thought and the prediced parallax error due to the Earth's movement around the sun was much smaller than older instruments could detect.
The much smaller changes in relative positions among the background stars were eventually detected and the evidence now supported the view that the Earth did move around the Sun.
Dark matter = a force greater than gravity = a freedom surface greater than coming together = a fixed observational plane for looking at a spread out object (or a distended object, e.g. a galaxy?). This fixed observational plane creates mixed perspectives on the distended object; evidenced by the appearance of parallax error (or alternative ways of looking at the distended object i.e. a kind of hologram of the object giving the observer a depth probe of the object due to his own distendedness (eg. the distendeness of looking at background stars from locations all along Earth's orbit of the Sun).
When observing the Milky way you get a distended observer (via his spread of observations along Earth's orbit of the Sun) UN-distended (via the Sun-Earth system's movement overthe years within the Milky Way) creating a disturbance in the correlation of stellar positions by the observer (a partial breakdown in parallax error as over the years the Earth-Sun system moves in the galaxy so upsetting the standard parallax-error model of stellar positions).
This mix of perspectives gives a precession effect which manifests itself in the appearance of a beating drum when you hold ("old") any ONE of the base-lines as a constant (the three alternative base-lines are: A map of stellar positions using Earth's orbit as a baseline (standard parrallax); the time-independent map of stellar positions from several years of Earth orbit baseline (the correction of standard parrallax); the time-evolving map of stellar positions (based on changes in stellar parallax observed over the years).
What is missing: with nowhere to turn (save the Magellanic clouds)to get free of these three baselines; astronomers have difficulty making sense of their observations. Something is disturbing them but what?
Enter "dark matter" as an idea. It looks like it will turn out to be human error.
"His team mathematically described our galaxy’s vibrations as a combination of three motions: a flapping of the disk’s edge up and down, an up-and-down motion like that of a drumhead, and a saddle-shaped oscillation."
If you mathematicalise (i.e. try to categorise)
this effect (of observing a distended object FROM a distended object, when both objects interfere with each other: e.g. using Earth-orbit baseline OVER YEARS to locate stars positions RE: EACH OTHER) you get
N E G A T I V E P A R A L L A X
or "blazing saddles" (pop-up advertisements that pinpoint YOUR location re: the background stars in this case?)
If you try to fix your "blazing saddles" you get
a globalised imaginary saddle (the up and down of the galaxies edge mimics the up and down curvature distribution of a saddle converging on a maximum deflection at the edge (in this case the up and down are inverted by the math-feedback loops I suspect); an up and down motion like a drumhead (due to the conservation of the mathematics: a saddling of imaginary globalisation i.e. of mathematics).
The THREE NOTES are:
the location of the stars NOW;
the location of the stars in the PAST;
the map of the stars relative to each other based on observed changes in the above two observations
and negative parallax
due to the fact that the Earth moves not only around the Sun
but that the Earth-Sun move among the stars
and neither move without regard to a third party e.g. the Magellanic clouds
But if you try to fix all three with the Magellanic Clouds as your reference point
you will get an exageration of the apparant gravitational influence of these clouds
but if you reconcile them with the Milky Way
you get a warp (an inversion: a partial apparent cancelling between negative and "positive" parallax)
i.e. an imaginary Milky Way
so the Milky way when imagined (distended by time) seems to flap (have conjunctivitis: have two centers rotating around each other)