This answer was provided by physicists Rhett Herman of Radford University in Radford, Va. and Shane L. Larson of Montana State University in Bozeman, Mont.
Dark matter is just what its name implies; it is matter (or mass) in the universe that we cannot see directly using any of our telescopes. Our telescopes see not only visible radiation (constituting the spectrum of colors that our own eyes can detect), but other types of radiation as well.
 Image: Royal Greenwich Observatory HIDDEN MASS. Optical telescopes see only the bright galaxies in this composite image. But the ROSAT satellite reveals that they are surrounded by a cloud of hot gas that emits x-rays. This gas accounts for some of the rotational effects on galaxies not accounted for by the optical image.
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Dark matter does not reveal its presence by emitting any type of electromagnetic radiation. It emits no infrared radiation, nor does it give off radio waves, ultraviolet radiation, X-rays or gamma rays. It is truly "dark." Cosmologists believe we can only see about 10 percent of the matter in the universe. Until they can accurately determine the mass of the universe, they will not know for sure whether it is expanding infinitely or will stop expanding at some point and collapse.
How, then, can we say with confidence that we know dark matter exists? The way in which dark matter reveals its presence to us is through the gravitational effect it exerts on luminous matter in the universe. ("Luminous" matter is the matter we can see with our telescopes.) The most obvious example of the gravitational effects of dark matter can be observed when looking at the rotation of galaxies.
To study galactic rotation, astronomers look at the emission line spectra of stars in each part of the galaxy. When the light from a star is observed using a diffraction grating or a prism, the starlight is separated into its true colors, in much the same way ordinary sunlight can be separated into the full rainbow of colors known as the visible spectrum.
The true colors constituting starlight separate into a series of light and dark lines in the visible spectrum, with each colored line corresponding to a specific wavelength of light. The specific wavelengths at which these lines occur are characteristic of the elements the stars contain. Thus, they can be used as an elemental "fingerprint" to identify a star's composition.
When a star emitting these line spectra is moving away from us, all of the wavelengths of the spectral lines are shifted to higher values than they would have been were the star stationary or moving side to side (neither towards nor away from us). All of the spectral lines are thus shifted towards the long wavelength part of the spectrum, or to the red end of the spectrum.
This shifting of the lines, known as a Doppler shift, towards the red end of the visible spectrum is the origin of the term "redshift." When a star has part of its motion towards us, the spectral lines are shifted to shorter wavelengths, or "blueshifted," towards the blue end of the spectrum. By measuring the shift in wavelength, researchers can calculate the precise speed of a star, either towards us or away from us.
When a galaxy is rotating, the starlight from stars on the side of the galaxy that is moving towards are blueshifted, while the starlight from the stars on the other side of the galaxy are redshifted. Thus, we can tell how fast and in what direction each individual star in the galaxy is orbiting about the center of the galaxy.
When stars orbit the center of a galaxy, their orbital speed is determined by the distribution of the mass contained within the galaxy. A graph showing the orbital speeds of the stars versus their distances from the center of the galaxy is known as the "rotation curve" for the stars in the galaxy.
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31 Comments
Add CommentI thought that this article was very one-sided. No dark matter has ever been seen and there is at least one viable alternate theory, which is not mentioned here. Dark matter might not exist at all: http://www.physorg.com/news85310822.html
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Reply | Report Abuse | Link to thisWhy main stream scientists want to believe that out there is only one kind of dark energy and dark matter and not many? (Or more appropriately named unknown instead of dark )
Costas Papaliolios
are we in global Warming or are we in another Ice Age....ahhhh science
Reply | Report Abuse | Link to thisThe article i just read was naming off different explanations for what the dark matter is. We should keep an open mind and remember that it might not be just one possibility. It might be most of them if not all.
Reply | Report Abuse | Link to thisThe article i just read had different explanations for what the dark matter is. We should keep an open mind and remember that it might not be just one possibility. It could be most of the possibilities if not all. (A lot of people associate darking and unkowing. That's probably why they call it dark matter.)
Reply | Report Abuse | Link to thisThe article i just read had different explanations for what the dark matter is. We should keep an open mind and remember that it might not be just one possibility. It could be most of the possibilities if not all. (A lot of people associate darking and unkowing. That's probably why they call it dark matter.)
Reply | Report Abuse | Link to thisThe article i just read had different explanations for what the dark matter is. We should keep an open mind and remember that it might not be just one possibility. It could be most of the possibilities if not all. (A lot of people associate darking and unkowing. That's probably why they call it dark matter.)
Reply | Report Abuse | Link to thisPerhaps dark matter is simply ordinary(or perhaps extraordinary) matter that is just too nebulous to detect, that fills the universe. If so, no matter how nebulous it may be, it would still cause some drag on objects moving through it. At low velocities it might be undectable, but at high velocities over a long period of time it should be significent.
Reply | Report Abuse | Link to thisSee The Pioneer Effect.
I have read several theories on dark matter and dark energy but was never really sold on the ideas presented. What if dark matter or dark energy is simply negative energy? Every article I read consists of tests to detect energy being emited. What if dark matter is the opposite? Could dark matter actually be simular to that of a black hole? (scaled down a bit ) Could dark matter be the oppisite of radiant (positive) energy? (yeng and the yang,,,,for every action there is an equal and oppisite reaction?) Are we able to detect energy before it is consumed by a black hole? and if so can this method be applied to the detection of dark matter?(that is ...detect energy consumed vs.energy emitted)Might the hadron collider provide more answers ??? and wouldn't it be cool if someone like me was the one that got it right ? lol
Reply | Report Abuse | Link to thisUnfortunately dark matter as based on galaxy rotation speeds is the result of nothing but a mathematical mistake.
Reply | Report Abuse | Link to thisWhen done properly, it is easy to find the mass distribution in the plane of a galaxy that will cause the measured rotation speeds, and no dark-matter spheres are needed. This should be obvious if one considers the problem from the other side.
Representing a galaxy in the standard way as a series of concentric rings in a plane, and knowing the gravitational effect of a single ring, the radial acceleration (and thus rotation speeds) can be easily computed for an arbitrary radial mass distribution in the plane of the galaxy. I call this the forward problem. This is easy using a computer with a finite number of rings, not easy at all if one tries to do it analytically with continuous mass distribution. However it has been correctly done analytically for a continuous exponential radial mass loading. (Freeman, ApJ 160, 811F, 06/1970).
For the reverse problem, mass distribution from rotation speeds, all the data needed are there and the solution is a simple matrix operation. With a reasonable estimate for the galxy thickness distribution, reasonable quesses can also be found for the density distribution. More details are in Nicholson, astro-ph/0309762 v2. The equations reduce to:
a(i,j) smd(i)=A(i), forward problem
smd(i)=ainv(i,j) A(i), reverse problem
where a(i,j)=acceleration effects of ring j of unit smd on ring i
A(i) = acceleration at ring i
smd(j)=surface mass density of ring j (mass per unit area)
The mathematical mistake can be seen in the analysis in the most quoted origin for dark matter, Van Albada et al (APJ 295, 8/15/1985). They assumed that since the surface light seemed to fall off exponentially with radius for galaxies that the mass distribution did the same, and thus Freeman's solution had to hold for all galaxies. When it was found that
measured rotation speeds at medium and large radii were much larger than Freeman's results. they needed some extra mass that would make up the difference. By assuming a series of spherical shells with adjustable densities, they could get approximations to the measured data. Thus dark matter was invented to get answers to a problem.
Energy emitted from Sun, is the result of Anti Gravitational Energy of the Planets in the Galaxy which is termed as Solar Food (Dark Energy). It is the Chemical Reaction of Solar Food that releases Solar Energy and the Sound that is released during this process is the Universal Music.
Reply | Report Abuse | Link to thisEnergy emitted from Sun, is the result of Anti Gravitational Energy of the Planets in the Galaxy which is termed as Solar Food (Dark Energy). It is the Chemical Reaction of Solar Food that releases Solar Energy and the Sound that is released during this process is the Universal Music.
Reply | Report Abuse | Link to thisEnergy emitted from Sun, is the result of Anti Gravitational Energy of the Planets in the Galaxy which is termed as Solar Food (Dark Energy). It is the Chemical Reaction of Solar Food that releases Solar Energy and the Sound that is released during this process is the Universal Music.
Reply | Report Abuse | Link to thisNow, I will readily admit that I am a relative neophyte when it comes to the physics of the Universe ... But is it possible that there is some property of the place that existed BEFORE our universe so luckily winked into existence (let's call this pre-universe place the Nothing... sorry Never-Ending Story fans) that pulls on anything that has mass? Since nothing ever had mass before the Big Bang or dimensional collision or whatever started this whole party, this Nothing never had anything to pull on. Then the singularity (or whatever) shows up, and the Nothing starts pulling as soon as soon as it appears. It isn't a rapid expansion at first (so much gravity fights so much Nothing-pull), but the Nothing-pull is too great and the expansion begins, accelerating everything into its Nothingness. Further, things that are nearer to the edge of the universe are being pulled at a greater acceleration than things closer to the center due to the increase of proximity to other forms of mass (and gravity) as you get closer to the center of the universe. Thus, the universe is increasing in its expansion into the Nothing forever.
Reply | Report Abuse | Link to thisEven with respect to the stars near the edge of individual galaxies, wouldn't these stars be pulled slightly away from the rotating center much like water on Earth causing the tides? (If this Nothing-pull is so great, once a star got too far from its own galaxy, could it fall under greater influence of the pull?)
Could someone please blow up this theory (or does it make some sense, just that nobody has ever looked at it this way). I know we're talking WAY outside the box here (hell, I'm talking about a potential force from outside the universe), but it does seem to me that the dark-matter theory could be a bit univer-centric... And I'm more of a fan of pull than push anyway (has there ever been any forces known to do any real pushing in the universe?). I'd love to hear the reasoning behind a theory that makes me believe that anything is really doing any pushing.
Further, if this dark matter really exists, would light be altered as it travels through it when we are observing objects out in the universe. Has anyone accounted for what must be the uniformity of dark matter that must exist (and would be hard to believe) for every view in every direction to be affected in the same way in what I think most would agree is not a homogeneous universe?
Keith Landry
Now, I will readily admit that I am a relative neophyte when it comes to the physics of the Universe ... But is it possible that there is some property of the place that existed BEFORE our universe so luckily winked into existence (let's call this pre-universe place the Nothing... sorry Never-Ending Story fans) that pulls on anything that has mass? Since nothing ever had mass before the Big Bang or dimensional collision or whatever started this whole party, this Nothing never had anything to pull on. Then the singularity (or whatever) shows up, and the Nothing starts pulling as soon as soon as it appears. It isn't a rapid expansion at first (so much gravity fights so much Nothing-pull), but the Nothing-pull is too great and the expansion begins, accelerating everything into its Nothingness. Further, things that are nearer to the edge of the universe are being pulled at a greater acceleration than things closer to the center due to the increase of proximity to other forms of mass (and gravity) as you get closer to the center of the universe. Thus, the universe is increasing in its expansion into the Nothing forever.
Reply | Report Abuse | Link to thisEven with respect to the stars near the edge of individual galaxies, wouldn't these stars be pulled slightly away from the rotating center much like water on Earth causing the tides? (If this Nothing-pull is so great, once a star got too far from its own galaxy, could it fall under greater influence of the pull?)
Could someone please blow up this theory (or does it make some sense, just that nobody has ever looked at it this way). I know we're talking WAY outside the box here (hell, I'm talking about a potential force from outside the universe), but it does seem to me that the dark-matter theory could be a bit univer-centric... And I'm more of a fan of pull than push anyway (has there ever been any forces known to do any real pushing in the universe?). I'd love to hear the reasoning behind a theory that makes me believe that anything is really doing any pushing.
Further, if this dark matter really exists, would light be altered as it travels through it when we are observing objects out in the universe. Has anyone accounted for what must be the uniformity of dark matter that must exist (and would be hard to believe) for every view in every direction to be affected in the same way in what I think most would agree is not a homogeneous universe?
Keith Landry
Yes dark matter (unknown to human) is there. we are always trying to negate the truth. God is invisible and his angles as well. In the same way there is a universe much bigger than that we know but that is invisible.......
Reply | Report Abuse | Link to thisPlease click here :
Reply | Report Abuse | Link to thishttp://lumi.chez-alice.fr/anglais/mystmass.htm
The more reasonable explanations are:
Reply | Report Abuse | Link to this1. The estimate of the mass of the galaxy is based on an erroneous assumption.
2. Our understanding of the red /blue shift is flawed (a little genuinely objective thought leads to the acceptance that the "standard" interpretation of a red or blue shift violates relativity).
Dark matter is an unfortunate "shortcut" that has arisen because the current scientific community is too arrogant and does not have the courage to challenge "accepted" concepts even when faced with an absurd result.
Also, I thought that this article was very one-sided. No dark matter has ever been seen and there is at least one viable alternate theory, which is not mentioned here. Dark matter might not exist at all:
Reply | Report Abuse | Link to thishttp://lumi.chez-alice.fr/anglais/mystmass.htm
Also I thought that this article was very one-sided. No dark matter has ever been seen and there is at least one viable alternate theory, which is not mentioned here. Dark matter might not exist at all:
Reply | Report Abuse | Link to thishttp://lumi.chez-alice.fr/anglais/mystmass.htm
Reply | Report Abuse | Link to thisSince speed involves time, is the data that is collected "time corrected" to account for the finite speed of light? It is probably a silly question, but I do wonder if the astronomers account for the time delay of radiation coming from different parts of a galaxy.
That sounds a lot like Aristotle's fifth element, the Ether. Scientists have been trying to figure out what that is for over 1800 years, and have only made a few discoveries such as how light is a transverse wave and not a longitudinal wave as previously thought.
Reply | Report Abuse | Link to thisBut who knows what makes up the rest of space? It could be nothingness, something unseeable, or possibly divine, but I doubt we will know within our lifetime.
Dark matter: The invisible force
Reply | Report Abuse | Link to thisThe temperature of space may hold the key to solve problem of missing matter
Dark matter, the "invisible" mass that scientists believe makes up a large part of the universe, can be explained as being mass that is at one end of a cooling process that all matter goes through.
This cooling theory is based on the idea that mass is a permanent entity, and the energy associated with mass is necessary only for matter that can be detected. Mass without such energy is the invisible dark matter.
The mass is all there -- but what kind of matter it becomes depends on temperature. Once matter reaches zero degrees Kelvin, it ceases to be detectable and becomes part of the missing mass that scientists cannot account for.
Imagine that the universe is composed of matter that is always in the process of moving from an invisible, dark-matter state into an energized and detectable -- state and then back again.
Through the natural process of cooling, the process where active matter radiates thermal energy till all energy charge is spent. The subatomic particles of an atom the protons, neutrons and electrons -- will use up the available energy and eventually reach 0K. When the matter reaches 0K, the mass of subatomic particles enter a neutral, undetectable state, having no particles in motion and therefore no thermal radiation.
In that environment, the mass of an atom remains. And while it would lack any thermal energy, it would maintain a gravitational force, and thus a potential source of Kinetic energy.
Instead of the term "dark matter," consider this to be a "neutral mass." It is mass in a stable state, that requires an occurrence involving extremely high energy acting upon the mass to set the subatomic particles in motion and recreate the atomic structure.
The neutral mass has a gravitational force and ability to attract other masses, producing kinetic energy with a cumulative effect that could be quite substantial.
A large concentration of this neutral mass would manifest as a black hole. Two black holes colliding -- bodies of dense, compacted neutral mass, each with enormous gravitational force and kinetic energy -- could yield an explosive action that would re-energize the atomic structure. That could take the form of new galaxies, which in turn are
the elementary building blocks of the universe.
Why not call it by its real name - aether. Dark matter is essentially the medium for the carrier of electromagnetism (aether) but dark matter has a very slight mass due to its proximity to matter which slows down the basic spin of pure aether.
Reply | Report Abuse | Link to thisThe Coney Island Green Theory explains Dark Matter, Dark Energy, quantifies a mass unit into a spatial quantity, combines the spacetime continuum with the mass-energy equation, solves Red Shift anomolies, and offers a solution to the Horizon Problem (though the later is not in the theory, one can extrapolate the solution out of it, namely that the temperature homogeneity is self promulgated from the MT=S manifestation), and yet, the initial postulate still remains "Believe it or Not"!
Reply | Report Abuse | Link to thisDark matter is discussed as ploy to avoide having the try to explain where is the source of all the energies that exist in the Universe. .
Reply | Report Abuse | Link to thismultiple universes as stated would account for the lack of the proposed gravitational force ... when the Milky Way finally completes its plunge into the super massive black hole at its center the singularity will expand releasing all of our matter as energy in a new space...which will collapse into H and eventually form another so called infinite universe and so on. There is as much proof for this as for dark energy and matter.
Reply | Report Abuse | Link to thisWhy is it so difficult for scientists to to admit, that if most of the energy available in the Universe is not visible or measurable , then this infinte quantity of energy must exist in a "potential" form. within the Aether of the Universe..
Reply | Report Abuse | Link to thisAll energies that are visible and measurable have already moved from the potential to the real condition, and further more this phenomena was a "Simultanious transition" therefore totally unobservable by any living creature.
The definitive answer to the title question is that the existence of dark matter is a hypothesis.
Reply | Report Abuse | Link to thisThe requirement for dark matter actually became established within the scientific community during the mid-1970s when astronomers, notably Vera Rubin, clearly demonstrated that spiral galaxies do not rotate like the Solar system. They had presumed that spiral galaxies must comply with the laws of planetary motion - specifically that Keplerian rotational curves (illustrating that the rotational velocities of planets orbiting the Sun diminish as their distance from the Sun increases) must also apply to individual stars within the discs of spiral galaxies.
That stars in the discs of spiral galaxies' velocities remained relatively flat as their distance from the galactic center increased was taken as convincing evidence that there must be some additional undetected mass accelerating the disc periphery and preventing peripheral stars from being expelled by their greater than expected velocities.
It should have been simply considered that, as Newton proved in his Principia, Kepler's equations, empirically derived from observation of the Solar system, were reliant on that system's distribution of mass (the Sun contains 99.86% of total system mass) and that disperse small body planets generally do not perturb one another's orbits.
From the perspective of a star at the periphery of a distributed mass spiral galaxy, its motion is not primarily determined by any center of mass located perhaps a hundred thousand light years away but more by much nearer comparably massive objects that almost surround it. The discs of spiral galaxies rotate as a loosely bound structure, not as individual stars independently orbiting any central mass (some large, well formed spiral galaxies do not even have a central bulge). Please refer to:
Feng and Gallo, (2010), "Rotating thin-disk galaxies through the eyes of Newton", http://arxiv.org/abs/1007.3778
The perceived requirement for galactic dark matter to explain their rotational characteristics very simply resulted from the misapplication of Kepler's equations describing specific characteristics of sparse planets independently orbiting the dominatingly massive Sun.
It seems that cosmologists' theories, in particular, have more recently become nearly completely dependent on their estimate that there is six time more dark matter in the universe that ordinary matter: it may one day turn around and bite them...
These gap filling dark matter equations seem to resemble another type of dark matter to me.... bovine fecal matter.
Reply | Report Abuse | Link to thisNick,
Reply | Report Abuse | Link to thisI'm sorry I'd overlooked your earlier comment, which as I understand is precisely correct.
Please refer to my recent brief commentary, which includes some more recent research reports building on Nicholson's methods:
"On not being the first to discover no galactic dark matter", http://www.sciencewithoutfiction.com/uploads/JDwyer.PDF
Excellent comment!