# What is dark matter?

1. Aug 19, 2009

### Baboon

What is dark matter? How was dark matter formed? Any replies would be greatly appreciated.

Last edited by a moderator: Aug 19, 2009
2. Aug 19, 2009

### fatra2

Simply put, dark matter is matter that is dark. hahaha!!!

A bit more details would probably be helpful. Let's start from the beginning. By looking at galaxies around us, we can see them spinning around a center. The speed of rotation depends on the amount of matter in the galaxy. The more matter the faster the spinning. Nothing too complicated up to now. Problem comes here. Calculating the rotational speed of galaxies from the only mass we see does not explain the speed observed in the telescops. Two solutions are possible: 1. our laws of phyiscs are wrong, 2. some matter is hidden somewhere. Of course, we could never imagine that we made a mistake drawing the laws of physics, therefore we had to look into matter that is not seen (therefore dark matter).

3. Aug 19, 2009

### Baboon

What about ...Dark matter is matter that cannot be detected by its emitted radiation but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. Estimates of the amount of matter in the universe based on gravitational effects consistently suggest that there is far more matter than is directly observable.

4. Aug 19, 2009

### fatra2

That's the beauty of this subject. By definition of "dark", dark matter does not emit any radiation.

Now the gravitational pull is the reason why we started wondering about dark matter. Since then, we came a long way. We know that there are many candidates that could explain this missing matter. First, let's not forget the massive objects (starts) at the end of the life, like white dwarfs, neutron stars, black holes don't emit radiation (or very few). They partly explain this rotational overspeed. Therefore, scientists attention turned to find what is the rest. You have the choice, between neutrinos, WIMP (weakly interactive massive particles) and many more. Since this is not my field, I can only give few details.

Your next comment might be on the neutrinos, thinking "how can such small particles explains missing mass of the universe???" My answer would be quite simple. Of course one does make that much of a difference in our Universe. But remember that more than $$10^{11}$$ particle pass through every kg of your body every second of your life.

To make the little story complete. These neutrinos have no electric charge (no electric field, and don't interact with matter, except for direct collision), very little mass (from what I remember less than 1/1000 the mass of the electron). They seem to be very good candidate for this dark matter.

Cheers

5. Aug 19, 2009

### George Jones

Staff Emeritus
The most massive neutrinos (there are three flavours) have mass less than 1/1000000 the mass of the electron (page 396 of the second edition of Introduction to Elementary Particles by David Griffiths).
Neutrinos likely account for only a small fraction of dark matter mass. Also, neutrino dark matter cannot account for structure formation in the early universe that leads to the galaxies and clusters of galaxies that we observe. Neutrinos move too fast to allow this to happen.

6. Aug 19, 2009

### fatra2

Thank you for the clarifications on neutrinos. I gave the numbers from the top of my head.

You might be right to say that neutrino account for only a fraction of the dark matter. From my understanding, we are just at the beginning of this field, and discoveries will most likely enlighten us in the near future.

Facts are that dark matter seems to be out there. We just need to find the right place to look for it.

7. Aug 19, 2009

### Baboon

Im the beginner in the physics and it is simply interesting to me
Forgive for a silly or simple question

tnx

http://www.u-n-i-v-e-r-s-e.com/the_Universe.html" [Broken]

Last edited by a moderator: May 4, 2017
8. Aug 19, 2009

### Heisenberg.

I have a question that regards to some of the comments posted above - According to string theory dark matter might possibly be a higher vibration of the superstring -Since string theory claims that us three dimensional beings can only see the lowest vibration of the superstring (e.g atoms, light) then dark matter might be the next set in vibrations - how popular is this string theory interpretation of dark matter, for I noticed it was not mentioned above? Is the reason neutrinos are so seemingly elusive to us the fact they have a higher vibration or are neutrions seperate from string theory altogether?

9. Aug 20, 2009

### Chronos

Dark matter is like neutrinos, we know its there but is mighty hard to directly detect. Most scientists doubt it is a Baskin-Robbins collage of neutrinos, rather suspecting it is a fundamentally different family of particles [I suspect there is more than one flavor, as is the case with neutrinos]. I usually generally avoid string discussions. The music is lovely, but, there are no lyrics.

10. Aug 24, 2009

### Hippasos

11. Aug 24, 2009

### Chronos

Expanding on [repeating?] what George said, neutrinos travel nearly at the speed of light. This is not conducive to large scale structure formation. Dark matter appears to travel around the same speed as ordinary matter.

12. Aug 24, 2009

### ideasrule

I haven't read the paper, but it seems to address why the galaxies are accelerating away from us, and not why galaxies have the observed rotation curve.

13. Aug 24, 2009

### George Jones

Staff Emeritus

14. Aug 24, 2009

### DaveC426913

It might be more more intuitive to see it as a form of matter (it could be much like protons and electrons for all we know) that simply does not interact with photons - neither absorbing them nor emitting them.

If it does not intereact with EM radiation, then it is invisible to all our sensory apparati yet still interacts normally with gravity.

15. Aug 24, 2009

### kldickson

What kind of technology would we need to positively detect dark matter and go beyond inferring its existence?

16. Aug 24, 2009

### DaveC426913

Why, a Dark Matter Detector of course.:tongue: (Go head. Ask what a DMD is and how it works.)

Seriously. You do realize that, since we don't know what it is or why we can't see it, there is no way of knowing what it would take...

17. Aug 25, 2009

### Chronos

Particle physics is the current search method. Even dark matter particles have a probability of interacting with normal matter, or other dark matter particles, if you observe a sufficient number of collisions.

18. Aug 25, 2009

### George Jones

Staff Emeritus
I think that if the LHC finds evidence of supersymmetry, the case for non-baryonic dark matter will be greatly strengthened.

19. Aug 25, 2009

### kldickson

Yes, dark matter particles don't emit electromagnetic radiation, but surely there are other ways of positively identifying them. What is known from their interactions with particles that do emit radiation?

20. Aug 26, 2009

### fatra2

The only way of identifying dark matter is through indirect effect. Like when a neutrino makes a direct hit with a nucleus, we can only measure the recoil of the nucleus and deduce that it was made by a neutrino.

If there would be a direct way of detecting dark matter, it would become "visible" in some way, and could not be called "dark" matter anymore.

Cheers

21. Aug 26, 2009

### azzkika

Am i correct in thinking the term 'dark' infers no emission of EMF whatsoever?

If that is correct, does that imply that dark matter does not interact with 'fields' the same as normal matter if at all?

Referring to the neutrino discussion, does the faster a particle travels have an increase to it's gravitational effect when passing another particle? I'm very amateur to physics, but if this was so, then maybe neutrino's could constitute more of dark matter if they are all travelling in the right direction.

22. Aug 26, 2009

### DaveC426913

Its behaviour is dictated by what we've given it as a nickname? :uhh:

Seems kind of the tail wagging the dog wouldn't you say?

23. Aug 27, 2009

### Chronos

'Dark' was coined in reference to its resistance to detection by means of kinetic reactions and EM emissions. DM is very much like neutrinos. It took us many years to confirm the existence of neutrinos, it will take us many more to detect DM particles. Neutrinos travel at nearly light speed, making them relatively easy to detect. DM does not, making it much harder to detect.

24. Aug 30, 2009

### _PJ_

I must admit, I've never liked the ideas of Dark Matter (or Dark Energy for that matter - pun unintentional) However, neutrinos perhaps account for some of it. It has occurred to me that gravtiational effects of some bodies may be 'concealed' by other matter, for examplethe concensus that there are black holes at the centre of many galaxies (perhaps all), including the Milky Way. The Milky Way centre has what is called the Great Attractor near its centre, and around this, many stars and their associated families no doubt are pullled into tight orbits. All this mass of which we cannot detect individually, only infer by the motion and what radiation is emitted, may therefore be 'hiding' greater mass behind it?

25. Aug 31, 2009

### Chronos

The 'bullet cluster' study is the smoking gun in the case for dark matter. See:
A direct empirical proof of the existence of dark matter
Douglas Clowe (1), Marusa Bradac (2), Anthony H. Gonzalez (3), Maxim Markevitch (4), Scott W. Randall (4), Christine Jones (4), Dennis Zaritsky (1) ((1) Steward Observatory, Tucson, (2) KIPAC, Stanford, (3) Department of Astronomy, Gainesville, (4) CfA, Cambridge)
http://arxiv.org/abs/astro-ph/0608407

We present new weak lensing observations of 1E0657-558 (z=0.296), a unique cluster merger, that enable a direct detection of dark matter, independent of assumptions regarding the nature of the gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray emitting plasma are spatially segregated. By using both wide-field ground based images and HST/ACS images of the cluster cores, we create gravitational lensing maps which show that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. An 8-sigma significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen.