[jack_ (OP)]

Peter is curious to know:

Has research been able to determine the structure or density of dark matter and does it interact with normal matter?

If so, it would seem collision would be frequent.The existence of dark matter detected by the effect its mass has on other objects…mainly galaxies if I understand the research correctly. Given that scientists estimate something like 75-85% of all matter is dark matter, clearly there is a lot of stuff out there.

Any insight you can give to help?

[Janus]

Dark matter would not be expected to interact with "normal" matter. (I really dislike using the word "normal" here. Since there is so much more DM in the universe, why couldn't it be the more "normal" state, while the matter we are made of is the outlier)
DM does not interact with electromagnetic radiation.  This means that it does not interact electromagnetically at all.
A collision between two hunks of "normal" matter really just involves the interaction of electromagnetic fields.  Remove those fields and they would just pass through each other like ghosts.   

Now since DM doesn't have this type of interaction, even with itself, it will not form that type of clumped up structures other matter does. The same electromagnetic interaction is what causes "normal" matter to stick together. This means DM forms low density clouds.  For example, the DM halo in which a typical galaxy is embedded, extends far beyond the limits of the galaxy proper.  Thus while there may be way more DM in that halo then there is other matter in the galaxy, the DM is spread out over a much, much, larger volume.   The galaxy itself is made up of structures that have a greater density than its average. Star systems are denser than the galaxy on a whole, and planets, Sun, etc are denser than the star systems in which they reside.
So, when push comes to shove, if you work out the density of DM in terms of overall mass divided into volume, it works out that the amount of DM spread out within the volume of the solar system is only about the same as a small asteroid.  So even if it did collide with the other matter in the solar system, it would not be a significant enough addition to notice.

[ConnorGunning]

Dark matter would not be expected to interact with "normal" matter. (I really dislike using the word "normal" here. Since there is so much more DM in the universe, why couldn't it be the more "normal" state, while the matter we are made of is the outlier)
DM does not interact with electromagnetic radiation.  This means that it does not interact electromagnetically at all.
A collision between two hunks of "normal" matter really just involves the interaction of electromagnetic fields.  Remove those fields and they would just pass through each other like ghosts.   

Now since DM doesn't have this type of interaction, even with itself, it will not form that type of clumped up structures other matter does. The same electromagnetic interaction is what causes "normal" matter to stick together. This means DM forms low density clouds.  For example, the DM halo in which a typical galaxy is embedded, extends far beyond the limits of the galaxy proper.  Thus while there may be way more DM in that halo then there is other matter in the galaxy, the DM is spread out over a much, much, larger volume.   The galaxy itself is made up of structures that have a greater density than its average. Star systems are denser than the galaxy on a whole, and planets, Sun, etc are denser than the star systems in which they reside.
So, when push comes to shove, if you work out the density of DM in terms of overall mass divided into volume, it works out that the amount of DM spread out within the volume of the solar system is only about the same as a small asteroid.  So even if it did collide with the other matter in the solar system, it would not be a significant enough addition to notice.

Geez, I didn't know any of that. That's exciting! Thank you!

[pensador]

Everything about Dark Matter is speculative. It might not even exist. Scientists know more about what Dark Matter can not possibly be than what it is. Dark Matter is predicted to exist in Random amounts around the outer edges of rotating galaxies to explain anomalies in the speed of rotation of galaxies without which galaxies would fly apart according to Einsteins Field Equations. It is also used to explain why light appears to be bent by unseen matter in regions of apparently empty space. https://www.britannica.com/science/dark-matter

If EFE are not accurate at long ranges then theories similar to MOND may be correct and Dark Matter may not exist in the quantities predicted.

Dark Matter is a useful thing to include in mathematical models when the models dont fit what is actually observed. ie Spiral Galaxies rotating faster than they should etc.

https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy

[pensador]

It seems another candidate AXIOMS for dark Matter has bit the dust https://phys.org/news/2019-03-dark-evidence-axions.html?utm_source=nwletter&utm_medium=email&utm_campaign=daily-nwletter
 

Physicists constrain dark matter and still don't know what it is.
https://phys.org/news/2019-03-physicists-constrain-dark.html?utm_source=nwletter&utm_medium=email&utm_campaign=daily-nwletter

Perhaps its a field and it doesn't exist 

[pensador]

And yet another candidate for dark matter bites the dust. This time tiny black holes https://www.sciencedaily.com/releases/2019/04/190402113042.htm

Perhaps Dark matter is just an effect of how gravity works, and not a detectable substance, as suggested by MOND

[pensador]

Another dark matter bites the dust, inelastic boosted dark matter, boosted by belief perhaps. Unfortunately, Lee and his colleagues were unable to detect IBDM signals in their data. https://phys.org/news/2019-04-inelastic-boosted-dark-terrestrial-detector.html?utm_source=nwletter&utm_medium=email&utm_campaign=daily-nwletter

[Janus]

And yet another candidate for dark matter bites the dust. This time tiny black holes https://www.sciencedaily.com/releases/2019/04/190402113042.htm

Perhaps Dark matter is just an effect of how gravity works, and not a detectable substance, as suggested by MOND

Too many observations rule out a "MOND only" explanation.  First, there is the Bullet cluster.  This is a collision between galaxy clusters.   What we see with it is a area where there is measurable gravitational lensing, but no visible matter associated with it.  This lensing is located where you would expect if dark matter had continued on at its normal speed after the collision while the visible matter had been slowed down by the collision.

Most recently, there have been the discovery of galaxies that show little to no behavior that indicates that they contain DM.  This is, in fact strong evidence for DM.   
 If the effects were due to gravity behaving differently than how we predict, then this would effect all galaxies in the same way.    If you have two galaxies that are identical in all other respects, they both would have to behave identically in terms of gravitational behavior.

However, if one galaxy behaves differently than the other in terms of gravity, then it must be due to that one galaxy containing something that we cannot see that makes it behave differently.    This argues for the DM hypothesis,  as unlike MOND theories which have to apply to all galaxies, there is nothing that says a galaxy has to have a certain given amount of DM.

[pensador]

Too many observations rule out a "MOND only" explanation

Yes MOND is an old theory, and one of the first to suggest Dark Matter might not exist. Many other theories exist based on Quantum mechanics string theory the holographic principle etc, none of which are requiring Dark matter. Emergent Gravity by Eric Verlinde has been tested against a claimed 30000 galaxies as of two years ago, and does not require dark matter to support galactic spin.

How is random amounts of dark matter or no matter introduced into galactic spin strong evidence for dark matter.

Emergent gravity doesn't appear to need dark matter at all. One theory predicting a random number is required to make an equation fit, and another not requiring any such random values to be introduced, might indicate one is a better model than the other.

It seems dark matter is something that is invoked every time something unexplainable using current theories comes up. The best wording ref Dark Matter is to call it the dark matter effect. ie there is an effect or something happening that cant be explained using current theories. Which could be a particle or a vector field related to galactic spin(hoffensteder) etc.

Dark matter is not required to explain galactic spin under emergent gravity. MOND was an early theory,that raised the possibility that some thing was wrong with current theory. Many respected theoretical physcists with Phd's also strongly suspect dark matter does not exist as I am sure you are aware.

However, if one galaxy behaves differently than the other in terms of gravity, then it must be due to that one galaxy containing something that we cannot see that makes it behave differently.    This argues for the DM hypothesis,  as unlike MOND theories which have to apply to all galaxies, there is nothing that says a galaxy has to have a certain given amount of DM.

Might it not also indicate that gravity does not work the way current theories indicate. The Dark matter effect is not conclusive evidence for dark matter.

[Janus]

Might it not also indicate that gravity does not work the way current theories indicate. The Dark matter effect is not conclusive evidence for dark matter.

No.  Even If the laws are gravity are different than what we assume they are, they still have to be consistent.  You can't have two galaxies both made of the same amount of baryonic matter arranged in the same way behaving differently from each other in terms of gravity no matter how you change the rules of gravity, because the rules have to apply equally to both galaxies. You can't have one galaxy behaving by one set of gravitational laws and another behaving according to an entirely different set of laws.  A successful model has to be able to explain all the observations and not just a selected sub-set.

[yor_on]

Let's see if I get you right Janus. Dark matter is a 'cloudy' but real substance only interacting gravitationally? Then again even 'photons/light' interact gravitationally so? Somewhat alike neutrons maybe?  Or something entirely different?

[pensador]

Might it not also indicate that gravity does not work the way current theories indicate. The Dark matter effect is not conclusive evidence for dark matter.

No.  Even If the laws are gravity are different than what we assume they are, they still have to be consistent.  You can't have two galaxies both made of the same amount of baryonic matter arranged in the same way behaving differently from each other in terms of gravity no matter how you change the rules of gravity, because the rules have to apply equally to both galaxies. You can't have one galaxy behaving by one set of gravitational laws and another behaving according to an entirely different set of laws.  A successful model has to be able to explain all the observations and not just a selected sub-set.

So if I have got this right.
All galaxies have between 1% to 5% dark matter applied randomly to make the maths fit.
Two Galaxies in the entire universe to date have been found that have little or no dark matter, therefore dark matter exists. 
http://www.astronomy.com/news/2019/03/ghostly-galaxy-without-dark-matter-confirmed
Could there be another explanation perhaps? Collision with other galaxies, losing a Black Hole, gaining a black hole, maths is wrong etc. Galaxies might collide and cause some galaxies to rotate in the opposite directions to their spiral arms,  is this an indication of dark matter or collision with other galaxies?