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Can anyone explain to me what dark matter is?

bringbackpluto wrote:Can anyone explain to me what dark matter is?

It is matter that we can't see, probably because it doesn't interact (or interacts only very weakly) with electromagnetic radiation. But it has mass, and therefore interacts gravitationally with normal matter, which is the primary reason for inferring its existence. We see things moving in ways that are best explained by the gravitational effects of something that we don't otherwise see.

If you don't want to have your question moved to the cafe', you need to post a related APOD like ...

http://antwrp.gsfc.nasa.gov/apod/ap060824.html

oops, too late

I'm no astrophysicist, but I am skeptical of something we can't see but which is required to make the math work out.

Seems more likely to me the math could be wrong.

As far as I know there is still no unified theory that explains everything we can observe.

-Noel

NoelC wrote:I'm no astrophysicist, but I am skeptical of something we can't see but which is required to make the math work out.

Seems more likely to me the math could be wrong.

When you shake a sealed box and hear something rattle, are you skeptical that there is something inside because you can't see it? Most scientists would be skeptical enough to admit the possibility that the box is empty and the sound is caused by some property of its material, but also recognize that the best explanation is probably that the box has something inside.

We do see dark matter, just not using electromagnetic radiation. Nobody's ever seen a photon, either. We infer their existence (and their properties) by "making the math work out" after observing effects we can't explain any other way. Most of our knowledge comes from inferences made by indirect observation.

Math is never wrong. Math is an internally self consistent system. If you have a problem with the observations that support the existence of dark matter, what you are really saying is wrong is gravitational theory, which is one of the best supported theories we have.

Chris Peterson wrote:

NoelC wrote:I'm no astrophysicist, but I am skeptical of something we can't see but which is required to make the math work out.

Seems more likely to me the math could be wrong.

When you shake a sealed box and hear something rattle, are you skeptical that there is something inside because you can't see it? .........

I think a better analogy would be.

When you shake a sealed box and it rattles, and someone tells you the rattle is a "Genie in a lamp", you are skeptical as to their explanation as to why the box rattles and not the fact that the box itself rattles.

You are skeptical of the explanation of the contents and not to the fact that the box contains something.

You are skeptical to the existance of "Dark Matter" but not to the fact that regular matter is being affected.

BMAONE23 wrote:I think a better analogy would be.

When you shake a sealed box and it rattles, and someone tells you the rattle is a "Genie in a lamp", you are skeptical as to their explanation as to why the box rattles and not the fact that the box itself rattles.

No, I don't think so. Nobody is saying with certainty what dark matter is. It is just loosely "something" that is invisible in the portions of the EM spectrum we have access to, and which exerts a gravitational influence which is measurable in several ways. Given this definition, very few scientists doubt that dark matter is real.

Beyond that, there are several proposals for what dark matter might be. Non-baryonic matter is the favorite, because it fits well with cosmological theory, but there are other possibilities as well. What's important is that all are testable, and so far none have been absolutely excluded.

Its going to be interesting to see if any positive detections come out from the Dark Matter detectors in the next few years, or if anyone detects gamma rays from the annihilations that some possible forms of Dark Matter are thought to undergo.

If we don't see any of these things then people are going to have to be even more serious about looking at gravity. There are some modifications of gravity that could potentially explain both Dark Matter and Dark Energy, that would tiidy things up very nicely.

"Dark matter" would make a good case study of the intricate interplay of observation, interpretation, and theory in modern astronomy.

IMHO, too much of what is written on the subject assumes that readers have a common understanding of the utterly essential role which physics plays in modern astronomy, let alone a common understanding of the relationship between physics and reality.

(This assumption has been shown to be false, numerous times, in many threads here in the Café; I think far too many of those beguiled by crank ideas, pseudoscience, and fringe science are much too unaware of the centrality of science theory to just about every aspect of their daily lives, much less astronomy or cosmology, to be able to begin thinking critically).

When I get time I intend to start a thread on this general topic, perhaps using 'dark matter' as a leading player.

Based on observations of galactic behaviour, and using the our calculations from the visible light and other spectrums to estimate what the mass of a galaxy would be, there seems to be too much gravity energy present to be accounted for by the visible matter we think we are seeing.

Dark Matter and Dark Energy were invented as factors to fill in the 'missing mass' needed to account for our gravity calculations.

Perhaps our gravity calculations are wrong, or our mass calculations. There are several assumptions involved in the postulation of DM & DE. I would suggest going over the assumptions again, and looking for errors, before unequivocally stating that DM & DE are necessary. It could also be the case of ordinary matter that is undetectable by us from this distance which is interspersed within or around a galaxy in a hovering layer.

DM & DE is where 'mainstream' imitates 'fringe'. Yet mainstream feels comfortable with its left shoe on its right foot.

Dark Matter and Dark Energy were invented as factors to fill in the 'missing mass' needed to account for our gravity calculations.

Not entirely, the amount of DM required to explain the behavior of galaxies and galaxy clusters is oddly exactly the same amount as is required to explain the shape of the CMB. Pretty good evidence that something is going on and has been the same thing for the entire lifetimeof the Universe.

The calculations to work out gravity are pretty simple, they are undoubtedly slightly wrong (we of course do not know where every hydrogen atom is), but they cannot be wrong by a factor of ten. Which leaves us with DM or a modification of gravity.

Thank you for the explanations, they have helped a great deal

astro_uk wrote:There are some modifications of gravity that could potentially explain both Dark Matter and Dark Energy, that would tiidy things up very nicely.

Interesting. It was my understanding that the concept of 'Dark Matter' was invoked to explain anomalously high orbital velocities of stars on the periphery of galaxies. I have no problem with envisioning vast clouds of DM (perhaps nonbaryonic matter) permeating galactic clusters.

What I do have difficulty with is a repulsive force associated with so-called Dark Energy!

Care to elaborate?

Well some modifications of gravity allow extra terms in the gravitional law, only at large distances do these modification cause measureable effects, one of which would be for gravity on the very largest scales to be smaller than expected, making it less able to hold back the expansion of the Universe, which to us would appear as an anomalous accelaration in the expansion. The trick is to get gravity to match over all these different scales, it has to behave normally on solar system scales, be stronger than expected on galactic and galaxy cluster scales, but smaller than expected on cosmological scales.

All of this is very difficult without destroying the amazing successes we have in matching GR to observations.

Your not alone in not liking Dark Energy, at a recent conference I was at most of the cosmologists had deep reservations about the actual reality of Dark Energy. But clearly there is something going on there and it has to be investigated.

There are several items already within this thread that I find interesting, not all having to do with the subject matter, but related because of use within the discussion.

The first is the concept of making the math work out. This is an essential to science. Indeed it was how Newton and Einstein developed their theories. There is no other way. The observed positions of bodies led to creating a reason for them to be in the position and the manner in which they moved from the position. Newton was not the first to develop his theory in this manner. Copernicus, for example.

Einstein made some modifications, if you will, and resulted in a better understanding of the force that holds here.

It is now time for someone, perhaps one of those who think that Dark Matter and Dark Energy cannot exist, to take it upon themselves to modify the theory of gravitation so that: a)Dark Matter does not exist, b)Dark Energy does not exist, c)the theory holds all over the known universe, d)all things presently explained by Dark Matter or Dark Energy are explained by the theory.

We, those of us with computer access to the World Wide Web, have at our fingertips more information(data) than either the three geniuses had and yet they produced intuitive concepts for the rest of the world. In fact, we may have too much data and be overwhelmed.

Postings in this subject have had little misconceptions(mine as well) that can grow outlandish in scope. It is the math that have created a need for Dark Matter and Dark Energy, and adjusting the math(making the math work) will be the only way to either prove the existence or non existence of this enigma.

Acceptance of the new theory will depend upon its application to newly discovered situations and or predicting other situations. As long as Dark Matter and Dark Energy are the only explanations for what has been detected then Occam's Razor shall apply.

Making the math work:
The cash register printout states that there should be $1,293.37 in the drawer
The cash count comes to $1,273.37
The math says: printout-cash count=missing money(if positive) or $1,293.37- $1,273.37= $20.00 is missing
The math shows the money missing but does not explain why but the following options do:
1) the cashier took it 2)customer shorted the cashier 3)cashier erred on giving change 4)Dark Energy took it 5)Dark matter swallowed it 6)Aliens got it
We don't get to use the razor on this.

Strange isn't it how 'Nobody's seen a photon' yet all we see are photons.

Gravity is not accurate within the solar system as the first spacecraft to exit the Solar system have shown.

This particular item makes me doubt the existence of Dark Matter and Dark Energy and lean towards a modified gravitational theory.

bringbackpluto wrote:Can anyone explain to me what dark matter is?

Well discussed here, in terms even I can understand!!

http://www.badastronomy.com/bablog/2007 ... moke-ring/

Michael Mozina wrote: This is like saying that the missing mass is a ring of invisible unicorns. Mind you it is not the unicorns themselves, but it's a map of their location using those gravitational distortions.....

Come on. You can't point to something 5 billion light years away and claim dark invisible unicorns did it.

It's nothing like that at all. We do see dark matter, just not by electromagnetic energy. But it is very clearly seen by its gravitational effects, not just in lensing but in directly affecting the motion of matter that is seen by EM. We have a very deep understanding of gravity, which has held up to hundreds of different tests. That makes it a fair tool for studying mass, whether visible in EM or not. People are biased because they have EM sensors built into their bodies, that's all. But that doesn't mean the Universe needs to make everything interact this way.

We already know about non-baryonic matter: neutrinos have extremely low interaction with normal matter (low enough to be dark matter candidates in some scenarios). They are also very difficult to detect. Just a few years ago they would have qualified as your "dark invisible unicorns".

The simple fact is that multiple avenues of observation point to the existence of dark, massive particles in the Universe. Such particles also fit well into existing cosmological models. As of now, they represent the best and simplest explanation to an observational puzzle.

(I've edited the [ quote ] tags, to provide the correct attribution*)

Michael Mozina wrote:

Chris Peterson wrote:It's nothing like that at all. We do see dark matter, just not by electromagnetic energy. But it is very clearly seen by its gravitational effects, not just in lensing but in directly affecting the motion of matter that is seen by EM.

I know that EM fields exist in nature, and I can play with them in a lab. I have never seen an astronomer produce even a single gram of dark matter, or any controlled experiment like a neutrino experiment where "dark matter" was shown to have any effect on matter or photons. It is therefore impossible to know if dark matter actually exist, or to verify or falsify any properties associated with "dark matter". If you could produce a gram of it, I'd be happy to let you speculate about there being tons of it out here. If you can't produce any, it's not any better than me attributing that lensings to "invisible potatoes".

(my bold)

I think it's getting close to time when this kind of mis-characterisation needs to cease, here in the Cafe.

Perhaps in a short while, when the thread discussing the nature of astronomy as a science - esp wrt the role of controlled experiments in earthly labs - has progressed a little more, it will become quite clear just how bad these mis-characterisations are, in terms of the nature of modern astronomy as a science.

To the point about invisible potatoes: if the missing mass were such, then the Earth would be being bombarded by meteors, with masses comparable to potatoes, element compositions comparable to those of potatoes, and with impact speeds of many dozens of km/sec.

That the Earth is, demonstrably, not being so bombarded is a powerful, observational, reason for rejecting your idea.

(There are, of course, many other such strong observational reasons to reject it).

We have a very deep understanding of gravity, which has held up to hundreds of different tests.

I have great faith in GR too, I just have no faith that it's the only influence on the universe.

Much as you may wish it were otherwise - or not - modern astronomy, as a science, does not include "Michael Mozina's faith in GR" as a critical component.

On the other hand, if you - or anyone else - can demonstrate consistency between good, pertinent observational results and some cosmological theory that GR is "the only influence on the universe", then let's see such a demonstration!

No, wait; cancel that ... the concodance LCDM cosmological models meet your requirements exactly ... they incorporate the Standard Model (of particle physics) as well as GR.

That makes it a fair tool for studying mass, whether visible in EM or not.

I will grant you that there is a high likelihood that "unidentified mass" is causing that effect. I have no evidence that this missing mass is due to dark matter, or invisible potatoes. Personally, I'm into iron suns.

Indeed.

Perhaps that's because you are unfamiliar with the relevant, high-quality observational results?

Or perhaps because you are unfamiliar with some of the key aspects of modern physics (the Standard Model of particle physics)?

Or perhaps because the cosmological principle which you, personally, use is inconsistent with that used by the scientists who are astronomers?

If you'd like an explanation of why it is exceedingly unlikely that anything more than a minor fraction of the observed DM could be potatoes or 'iron suns', then please just ask.

People are biased because they have EM sensors built into their bodies, that's all. But that doesn't mean the Universe needs to make everything interact this way.

I could just as easily suggest that some people are biased because they have GR centric views of the universe and they refuse to consider alternative ideas to "dark" things because they've been indoctrinated to believe in "dark" stuff.

You could, of course, suggest just that.

However, I think you'd have a very difficult time indeed trying to defend such an outrageous, almost libellous, assertion.

Fortunately, quite a number of very knowledgable, very capable scientists have had a crack at 'alternative ideas to "dark" things' (perhaps because they've not been indoctrinated to believe in "dark" stuff?); unfortunately, none of those ideas has come anywhere near being able to account for the range of cosmologically relevant observations as well as LCDM models can.

[snip]

Such particles also fit well into existing cosmological models. As of now, they represent the best and simplest explanation to an observational puzzle.

How do such particles fit into solar density calculations?

Very well indeed ... the estimated, local, density of (non-baryonic) DM is way below that which could be detected in any observations of the Sun, using current technologies.

IIRC, there have, in fact, been several papers on just this general topic ... only if DM were baryonic would it be detectable within the solar system (potato-mass meteors arriving on hyperbolic orbits, for example).

*Would you mind editing your post Michael, to correctly attribute the text you quote?

Michael Mozina wrote:I know that EM fields exist in nature, and I can play with them in a lab. I have never seen an astronomer produce even a single gram of dark matter, or any controlled experiment like a neutrino experiment where "dark matter" was shown to have any effect on matter or photons. It is therefore impossible to know if dark matter actually exist, or to verify or falsify any properties associated with "dark matter". If you could produce a gram of it, I'd be happy to let you speculate about there being tons of it out here. If you can't produce any, it's not any better than me attributing that lensings to "invisible potatoes".

It is impossible to say that either matter or energy even exist. We simply find them convenient for explaining our observations. The Universe is full of things we've never touched directly, or had in the lab. That doesn't mean that we can't propose that their existence is likely. Nobody has ever created a gravitational red shift in the laboratory. Do you consider its existence to be on the same level of probability as invisible potatoes?

I will grant you that there is a high likelihood that "unidentified mass" is causing that effect. I have no evidence that this missing mass is due to dark matter, or invisible potatoes. Personally, I'm into iron suns. :)

The point is, most people consider dark matter to be the best available explanation.

I've seen real evidence of real neutrinos. I'll be happy therefore to let you use them to explain some of that "unidentified mass". Got any experiments that show "dark matter" has an effect on mass like you can with neutrinos?

That's not the point. Neutrinos just go to show that non-baryonic matter is real. They demonstrate that there is matter that is extremely difficult to detect, which took years to find (well, apparently to find) after being predicted.

No, their existence was "predicted" by particle physics and known laws of nature. Even before we could detect them, we had evidence they existed based on controlled scientific tests in a lab.

And the same now holds for dark matter.

That is distinctly different than trying to attribute an uncontrolled observation of something that is actually 5 billion light years away and claiming "invisible stuff" did it.

Again, it isn't invisible. It just doesn't interact with EM. We see it in the "light" of gravity.

How "massive" is massive here? Can you define it's physical size? How come if normal matter only accounts for about 4% of the universe, none of the density calculations for suns even mention the other 96% of the universe? No solar density influences from dark matter? None from dark energy either? You don't find that sort of hard to swallow after looking at that missing mass and seeing how widespread it is?

Not really. Your numbers are in units of energy. They only say that most of the Universe is in a form other than matter, so density as I think you are applying it doesn't make sense. As far as actual visible matter is concerned, it obviously isn't homogeneous. So why expect that of non-baryonic matter? No, it doesn't bother me that we don't find it in stars, or perhaps more likely, we don't find it in sufficient amounts to detect by any methods we've yet developed.