[Recrudesce (OP)]

How do we know that dark matter is there ?
can we see it, detect its chemical signature, or its gravity ?
 my understanding is No, but it has to be there because someone said so, please clarify this for me !

[Bored chemist]

You can't see the wind, but you can tell there's something there, because the flags wave and the leaves flutter on the trees.

We can't see dark matter but we know it is there because something must be stopping spiral galaxies tearing themselves apart as they spin. That something has   to be gravity and so there must be something with mass there.
But we don't know what it is. The only thing we know is that it's not ordinary matter.

[PmbPhy]

We can't see dark matter but we know it is there because something must be stopping spiral galaxies tearing themselves apart as they spin. That something has   to be gravity and so there must be something with mass there.
But we don't know what it is. The only thing we know is that it's not ordinary matter.

That is incorrect. It's mass that keeps spiral gravity together. Dark matter is what contributes to the gravity but is unaccounted for by the matter that can be seen from the light from the stars in galaxies. The source can be things like brown dwarves, black holes and particles that only act through gravity.

[Bored chemist]

It's mass that keeps spiral gravity together.

Pardon?

[Bored chemist]

The source can be things like brown dwarves, black holes and particles that only act through gravity.

You got it right in the end
 it's the "particles that only act through gravity" bit.
It isn't the other stuff like brown dwarfs which are made from ordinary matter.
.

https://en.wikipedia.org/wiki/Dark_matter

[Bogie_smiles]

Here is a nice video I posted yesterday on that topic:
https://youtu.be/P1dd8vVp1Vw

[Zer0]

https://www.nasa.gov/home/hqnews/2006/aug/HQ_06297_CHANDRA_Dark_Matter.html

Just incase if anyone is hesitant or simply lazy to click on the link above :

RELEASE : 06-297
 
NASA Finds Direct Proof of Dark Matter
 
Dark matter and normal matter have been wrenched apart by the tremendous collision of two large clusters of galaxies. The discovery, using NASA's Chandra X-ray Observatory and other telescopes, gives direct evidence for the existence of dark matter.

"This is the most energetic cosmic event, besides the Big Bang, which we know about," said team member Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

These observations provide the strongest evidence yet that most of the matter in the universe is dark. Despite considerable evidence for dark matter, some scientists have proposed alternative theories for gravity where it is stronger on intergalactic scales than predicted by Newton and Einstein, removing the need for dark matter. However, such theories cannot explain the observed effects of this collision.

"A universe that's dominated by dark stuff seems preposterous, so we wanted to test whether there were any basic flaws in our thinking," said Doug Clowe of the University of Arizona at Tucson, and leader of the study. "These results are direct proof that dark matter exists."

In galaxy clusters, the normal matter, like the atoms that make up the stars, planets, and everything on Earth, is primarily in the form of hot gas and stars. The mass of the hot gas between the galaxies is far greater than the mass of the stars in all of the galaxies. This normal matter is bound in the cluster by the gravity of an even greater mass of dark matter. Without dark matter, which is invisible and can only be detected through its gravity, the fast-moving galaxies and the hot gas would quickly fly apart.

The team was granted more than 100 hours on the Chandra telescope to observe the galaxy cluster 1E0657-56. The cluster is also known as the bullet cluster, because it contains a spectacular bullet-shaped cloud of hundred-million-degree gas. The X-ray image shows the bullet shape is due to a wind produced by the high-speed collision of a smaller cluster with a larger one.

In addition to the Chandra observation, the Hubble Space Telescope, the European Southern Observatory's Very Large Telescope and the Magellan optical telescopes were used to determine the location of the mass in the clusters. This was done by measuring the effect of gravitational lensing, where gravity from the clusters distorts light from background galaxies as predicted by Einstein's theory of general relativity.

The hot gas in this collision was slowed by a drag force, similar to air resistance. In contrast, the dark matter was not slowed by the impact, because it does not interact directly with itself or the gas except through gravity. This produced the separation of the dark and normal matter seen in the data. If hot gas was the most massive component in the clusters, as proposed by alternative gravity theories, such a separation would not have been seen. Instead, dark matter is required.

"This is the type of result that future theories will have to take into account," said Sean Carroll, a cosmologist at the University of Chicago, who was not involved with the study. "As we move forward to understand the true nature of dark matter, this new result will be impossible to ignore."

This result also gives scientists more confidence that the Newtonian gravity familiar on Earth and in the solar system also works on the huge scales of galaxy clusters.

"We've closed this loophole about gravity, and we've come closer than ever to seeing this invisible matter," Clowe said.

These results are being published in an upcoming issue of The Astrophysical Journal Letters. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center, Cambridge, Mass. For additional information and images, visit:
http://chandra.nasa.gov

✌
[EDIT]
Courtesy & Credits - NASA
(geez I forgot, sorry) 🙏

[PmbPhy]

https://www.nasa.gov/home/hqnews/2006/aug/HQ_06297_CHANDRA_Dark_Matter.html

Cool. I worked on that telescope. I was on the team that calibrated the CCDs.

[guest45734]

How do we know that dark matter is there ?
can we see it, detect its chemical signature, or its gravity ?
 my understanding is No, but it has to be there because someone said so, please clarify this for me !

Your question might be better worded " Why do we suspect dark matter is there"

In order for general relativity to be correct an additional form of matter as yet undetected must exist in the universe. Dark matter is required to explain why the outer parts of galaxies move so fast but dont fly off. Observed effects such as gravitational lensing are attributed to dark matter in some galaxies. NO ONE KNOWS for sure exactly what dark matter is or even if it exists.

[geordief]

http://www.bbc.com/news/science-environment-43543195

They may  have found a Galaxy with almost no Dark Matter.

[Bill S]

Dark Matter Goes Missing in Oddball Galaxy

https://www.laboratoryequipment.com/news/2018/03/dark-matter-goes-missing-oddball-galaxy?et_cid=6301598&et_rid=517749120&type=cta&et_cid=6301598&et_rid=517749120&linkid=https%3a%2f%2fwww.laboratoryequipment.com%2fnews%2f2018%2f03%2fdark-matter-goes-missing-oddball-galaxy%3fet_cid%3d6301598%26et_rid%3d%%subscriberid%%%26type%3dcta

"This invisible, mysterious substance is the most dominant aspect of any galaxy. So finding a galaxy without it is unexpected. It challenges the standard ideas of how we think galaxies work, and it shows that dark matter is real: it has its own separate existence apart from other components of galaxies."

[Bogie_smiles]

It seems that dark matter is most obviously needed to reconcile missing matter around to outer reaches of spiral galaxies. This ghost galaxy, that is said to have no dark matter, is not a spiral, but more of an evenly distributed clump of stars with no arms. I wonder if that plays into the picture when observing galaxies and equating their dark matter composition?

[guest45734]

It seems that dark matter is most obviously needed to reconcile missing matter around to outer reaches of spiral galaxies.

Dark matter is required to reconcile relativity with what is observed in spiral galaxies. It might be that relativity is not 100% correct, and Dark matter might not actually exist. Relativity correctly predicts the movement of the stars and planets in the inner regions of galaxies where space is more curved/stretched without the need for dark matter.

Theories are being worked on that do not need dark matter for instance https://phys.org/news/2016-11-theory-gravity-dark.html , https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory , MOND, etc etc.

Dark matter might not exist. No one knows what it is.

[Bogie_smiles]

I agree, having speculated about it out in New Theories a couple of times.

[guest45734]

I found this related to the above link from the bbc http://www.bbc.com/news/science-environment-43543195

I agree, having speculated about it out in New Theories a couple of times.

Thanks for the heads up, I will have a look under new theories and see what has been speculated on dark matter.

EDIT: I have just been looking for the mathematical derivation for the proof that dark matter must exist using either relativity or newtonian ideas, and stumbled across this link using relativity, whereby dark matter is NOT required to explain the movement of the outer parts of galaxies.

https://arxiv.org/ftp/arxiv/papers/1503/1503.07440.pdf 

This explanation doesn’t modify the quantities of
observed matter and doesn’t modify general relativity. To
demonstrate the capacity of the general relativity to explain dark
matter, we are going to use, in our paper, the native metric of
linearized general relativity (also called gravitoelectromagnetism)


 


 

[Janus]

It seems that dark matter is most obviously needed to reconcile missing matter around to outer reaches of spiral galaxies. This ghost galaxy, that is said to have no dark matter, is not a spiral, but more of an evenly distributed clump of stars with no arms. I wonder if that plays into the picture when observing galaxies and equating their dark matter composition?

The thing to keep in mind is that the bright spiral arms of spiral galaxies are not due to there being a great deal more material there, just brighter stars on average.  They are regions of more active star births. The brighter and more massive of these new stars also have the shortest life spans.  They burn out quickly (on the galactic time scale).  The "gaps" between the spiral arms are actually full the remnants of these burned out stars and smaller dimmer stars which have longer life spans.  The gaps look "dark" only in comparison.  This is much like how sunspots look black against the disk of the Sun, but in terms of absolute light output, are actually blindingly bright.

[Bogie_smiles]

The thing to keep in mind is that the bright spiral arms of spiral galaxies are not due to there being a great deal more material there, just brighter stars on average.  They are regions of more active star births. The brighter and more massive of these new stars also have the shortest life spans.  They burn out quickly (on the galactic time scale).  The "gaps" between the spiral arms are actually full the remnants of these burned out stars and smaller dimmer stars which have longer life spans.  The gaps look "dark" only in comparison.  This is much like how sunspots look black against the disk of the Sun, but in terms of absolute light output, are actually blindingly bright.

The dark matter speculation is that the arms seem to be "held" in, and not spiraling off into deeper space, implying that there is matter that is as yet unseen, within the galaxy, restraining their motion outward.

As for there being more active star births of large bright fast burning stars that die and explode, leaving their remnants in those outer portions of the spiral galaxies, I find that quite possible. However, it is also possible that in addition to that, the entire galaxy is formed from the remnants of an earlier supernova that occurred during an earlier round of huge fast burning hydrogen stars.

So maybe the new star births in the arms is a factor of a gradual accumulation of those earlier supernova remnants gravitationally drawn in from a large remnant field dating back in time. The gaps between the arms would have lots of those older remnants. The arms and their gaps would also seem to form from the revolving motion of the initial supernova remnant field. The earlier supernova could have imparted a revolving action to the debris if the remnant black hole was revolving relative to the surrounding mass in the other galaxies whose positions are more "fixed" relative to the particular spiral galaxy in question.

[Bogie_smiles]

Note the new article in Wired, which adds to the dark matter debate
https://www.wired.com/story/whisper-from-the-first-stars-sets-off-loud-dark-matter-debate/
I mentioned that there were some posts about dark matter in my New Theories thread, and then this came out today in Wired. I know I can't speculate in this sub-forum, but there are some conclusions that can be drawn that are not yet generally accepted. I will post in my speculations thread in New Theories about how this article ties in with the video I posted above.