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It has been observered that spiral arm formation galaxies exist. This is a problem because due to the laws of gravitation, stars close the galactic center should orbit with a much higher angular velocity than stars at the distant edge of a galaxy, and so any structure, spiral arms included, should smear out after one or more revolutions of the stars and become a disk galaxy.

In order to explain this unexpected observation, cosmologists hypothesise that something is causing the stars to all orbit at constant angular velocity, so as to keep the structure, spiral arms, intact. The name given to the "something" that causes this is Dark Matter.

There are a number of problems with this hypothesis. Specifically, when two galaxies are near each other, the "matter" that is maintaining constant angular velocity in one galaxy should also be having an effect on the neighbouring galaxy, yet we see no such effect.

Similarly, if the "dark matter" is really a field effect, then the field that causes constant angular velocity for stars in one galaxy should also be having an effect on the neighbouring galaxy, yet again, we see no such effect.

So, given this, is it sensible to consider the term "dark matter" to actually mean matter, or even a field effect. Or is it sensible to consider "dark matter" to be some as yet unidentified effect?

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    $\begingroup$ Dark matter is not a field effect. I don't know what you are even trying to say. Dark matter is matter we can't see with our current technology. There is many forms of dark matter with their own fields of research. Dark matter differs for modified newtonian dynamics specifically because it states that there is matter there, we just can't see it as opposed to there being difference physical affect at play at the scale of galaxies. $\endgroup$
    – A. C. A. C.
    Commented Nov 3, 2017 at 17:51
  • $\begingroup$ What i am trying to say it that the reason for the existance for the term "dark matter" is purely due to the apparent constant angular velocity of stars orbiting a galactic center. In other words, "dark matter" is the label given to whatever is causing the constant angular velocity. My question is, if dark matter is actual matter, then why dont we see it affecting neighbouring galaxies, when they are in close proximity. Similarly, if dark matter (its just a label after all) is a some kind of field effect, then also why does it not affect neighbouring galaxies ? $\endgroup$
    – Tim
    Commented Nov 3, 2017 at 17:55
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    $\begingroup$ Far as I know dark matter is just matter that we can’t see. It does add to the mass of the galaxy and it does affect other galaxies as they are drawn towards each other. $\endgroup$
    – Bill Alsept
    Commented Nov 3, 2017 at 17:59
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    $\begingroup$ The dark matter hypothesis fits galactic clusters too, see en.wikipedia.org/wiki/Dark_matter#Galaxy_clusters also rotational curves of galaxies en.wikipedia.org/wiki/Galaxy_rotation_curve $\endgroup$
    – anna v
    Commented Nov 3, 2017 at 18:10
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    $\begingroup$ Tim, do you think quantitatively at all? You say, two effects should exist, "we see no such effect". Have you tried to estimate what the magnitude of these effects should be, according to dark matter theory? $\endgroup$
    – Mitchell Porter
    Commented Nov 3, 2017 at 19:39

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What made you believe that dark matter doesn't have any effect on neighboring galaxy?

Secondly, dark matter and dark energy are just placeholder names that we don't know anything about. We know whatever it is, its some sort of matter/energy because that's the closest thing we could compare it to.

Also, consider this: We can only observe less 5% of our universe. And you are claiming that this thing what constitutes more than a quarter of our universe has no effect on the rest of it?

Why do we call it dark matter? Matter? because it interacts with our "visible matter" through gravity. We calculated gravity of all the visible matter and realized that 85% of gravity is still missing. We figured that since gravity comes from matter as it bends the fabric of space-time. Maybe there is another matter that bends the fabric as well but we can't observe it (hence dark).

There are plenty of effects of dark matter that we observe and "galaxy's spiral-arm" thing is one of the many effects of dark matter.

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  • $\begingroup$ That is exactly the point I am making, that dark matter is just a "label" to for whatever causes constant angular velocity in stars around a galactic center. And, as you mentioned, that dark energy is just a "label" for whatever is causing in increase in accelaration of galaxies away from each other. I agree entirely. They are just placeholders. Which is why, I ask, why is dark matter called "dark matter". The point being to hopefully educate. $\endgroup$
    – Tim
    Commented Nov 3, 2017 at 18:53
  • $\begingroup$ @Tim The statement that "we can only observe less than 5% of our universe" is incorrect. The correct statement is that the theories that predict "additional 95%" (which we don't see) are not supported by the observations and therefore must be dismissed. $\endgroup$
    – safesphere
    Commented Nov 3, 2017 at 19:38
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    $\begingroup$ What are you talking about? Theories not supported by evidence? We can observe the gravitational effect of dark matter. From gravitational lensing to cosmic microwave background to a whole bunch of other observational effects. We don't have any explanation why the universe is expanding with an ever increasing acceleration. The gravity of observable matter (including black holes) is around 15% of the actual gravity we observe in the universe. Read any book written by mainstream physicists of this century and you will find that I am not wrong according to current theories. $\endgroup$
    – LostCause
    Commented Nov 3, 2017 at 19:59
  • $\begingroup$ Updated my answer. Hopefully, now you know we it's called dark matter. But call that thing "Frankenstein Particle" or "Unicorn's horn" and no scientist will argue with you if you can bring them one step closer to understanding it, they might even give a noble prize. $\endgroup$
    – LostCause
    Commented Nov 3, 2017 at 20:10
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    $\begingroup$ @Tim "Dark," all by itself is a label. Astronomers have been applying it since the 19th century to anything in the sky that they can infer, but not actually see. en.wikipedia.org/wiki/Dark_matter#Early_history $\endgroup$
    – Solomon Slow
    Commented Nov 3, 2017 at 20:19
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It's dark because it doesn't emit light. I don't know what you mean by we don't see an effect of DM on other galaxies. There is definitely observational evidence for dark matter within galaxy clusters. For instance, one sees the same sort of rotation phenomena as you see within the galactic arms (the example you sighted). The luminous mass vs. gravitational mass ratio one finds that way is in agreement with what one measures from gravitational lensing.

There are plenty of examples that are totally consistent with DM at larger than galactic scales.

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Dark matter is stuff we still do not understand yet. That is the reasoning for the term "dark". Of course dark matter is seen affecting other galaxies during galactic collisions. Dark matter is "matter" in the sense it has a gravitational effect. By field, if you mean a quantum field then it sort of makes sense. This is because according to quantum field theory all particles have their respective fields. However that does not mean it will have field like behavior. It will not really change the properties of those dark matter particles and it does not really change anything. It will still have its particle like behavior.

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  • $\begingroup$ When I say seen I mean "seen" in the sense we have seen dark matter's gravitational effect on regular matter. We can not see dark matter. Dark matter does not interact with electromagnetic waves. $\endgroup$
    – Roghan Arun
    Commented May 6, 2020 at 14:33
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Why is dark matter called dark matter?

Really this is a historical misnomer. Kapteyn and others found already in the 1920s that the dynamics of stars in the Milky Way can't be explained based on the gravity that we expect due to the observed matter. They thus inferred that there would need to be additional, unseen matter. If you're behind an eyepiece on a telescope, it is rather natural to call such matter "dark", in the sense of "not visible". Dark matter also does not emit e.g. thermal radiation.

Unfortunately, "dark" would imply "black" and that is an incorrect label. Evidence for dark matter has since accumulated across astrophysics and cosmology. In particular observations of the Cosmic Microwave Background radiation tell us that this dark matter does not interact with light. Therefore, a more proper name would be "transparent matter".

So why is dark matter still called dark matter? Historical reasons. Or, perhaps simply because this label captivates the imagination... ;)

Specifically, when two galaxies are near each other, the "matter" that is maintaining constant angular velocity in one galaxy should also be having an effect on the neighbouring galaxy, yet we see no such effect.

But we do. Observations of tidal streams are in the process of becoming a workhorse to learn about the properties of dark matter.

... the field that causes constant angular velocity for stars in one galaxy should also be having an effect on the neighbouring galaxy, yet again, we see no such effect.

But we do. For example, orbits of dwarf galaxies around the Milky Way are in agreement with the inferred total mass of the Milky Way, i.e. including dark matter.

Is it sensible to consider the term "dark matter" to actually mean matter...

Yes, very.

...or is it sensible to consider "dark matter" to be some as yet unidentified effect?

I'm not sure I understand the subtle distinction that is made here, but happy to consider it if you could clarify a bit what you mean.

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