Can some please explain what "Dark Matter" is in a simple way for me?
447237
Dark matter provides the gravitational force that prevents galaxies being ripped apart by their angular momentum.  We have no way of detecting it, except from the bending of light around it.
75146
There currently is no explanation for dark matter just some guesses based on observations.  Some of the observations are the following: \n\n1.  Gravitational lensing from massive galaxies is stronger than expected based upon the average "visible" mass of many galaixes.\n\n2.  Galactic rotation curves do not follow Keplerian motion at distances far from the center of spiral galaxies which seem to imply that there is unseen matter in a halo around most galaxies.  It has been recently proposed that cetain general relativistic treatments of mass distributions of galaxies could explain rotation curves with out the use of dark matter.  It appears that this latter theory is beginning to gain acceptance (see paper under sources).   \n\nIf dark matter does exist then it is matter that doesn't interact (or weakly) via photons (the electromagnetic force), likely not made of electons, but does interact gravitiational.  Nothing currently is known to have these properties.  Objects like brown dwarfs, black holes and white dwarfs were believed to contribute to dark matter however it has been shown that the space density of these object don't add up to a signifcant amount.  \n\nOther theories include MACHOs (Massive Astrophysical Compact Halo Objects; origin unknown) or the theoretical particles called axions and WIMPs (Weakly Interacting Massive Particles).
918848
According to general relativity, the matter inside a black hole collapses to a single point: a "singularity".  At this point, the mass density (ρ = m/V) is undefined (because the volume of a single point is zero). \nWe have a lot to learn about extreme cases such as this.
1052554
Well, not speculating on whether dark matter is present or not, we know, from observation that there is something capable of altering the rotation curves of galaxies, causing gravitational lensing, affecting the flow of interstellar gases (including one case, VIRGOHI21, where a bunch of diffuse hydrogen in a region of no stars whatsoever is behaving as though it had a rather substantial mass despite the complete absence of any stars), and altering the large-scale structure of the universe, in precisely the same way that gravity does, and say, electromagnetic forces do not. We know that this mysterious gravity-like force is being exerted by objects that do not emit or absorb light, because we would be able to see it if it was. The simplest explanation for this phenomena is that it is in fact gravity, and that this gravity is being caused by the presence of large amounts of matter that we cannot see - i.e. dark matter. I suggest that you look at the wikipedia articles on this subject for more details.
771754
As others have said, it just doesn't want to come out and play.  Believe about 4% of the mass/energy in the universe is normal matter, and about 23% is dark matter.  Dark energy is the major player but is not related to dark matter.  They have completely different effects.\n\nEri is probably referring to the recent "photo" of two colliding galaxies.  I believe that analysis of gravitational lensing phenomena was incorporated in the image to illustrate the distribution of mass.  The image indicated most of the mass in each galaxy had jumped ahead of the visible mass, which was a new way of seeing dark matter.  However, lensing phenomena is still relying on gravity for detection.
380500
It's the matter in galaxies that provides enough gravitational force to prevent them from being flung apart by their angular momentum.  It's called dark because we can't see it
561306
you should read more into string theroy and the M theory.  they are both very interesting.  it is hard to wrap your mind around but  this is just one universe that goes along a membrane of universes that goes on infinetely.  the science channel often has show that explin all this much better and they make it pretty easy to understand.
476348
Ask your friend if he knows about "dark matter". \n\nAlmost all the astronomers and most of the physiscists believe nowadays that only 4% of the Universe is made of "normal matter", that is made of neutrons, protons, electrons, photosn, etc (all the aprticles contained in the standard model of Particle Physics). The rest of the Universe is believed to be made of something whose nature is still not known, 26% of this "something" is called dark matter, the name is because it doesn't interact via electromagnetism, therefore, it can not produce, or be affected by light of any frecuency, it is then dark in the sense that no instrument based on light can detect it. The presence of dark matter however affects massive bodies by its gravitational influence, in the last decades its existence has been inferred in objects of very large size, like galaxies and cluster of galaxies, there are many indirect observations that sustain the existence of dark matter, the most stronger is probably the observed rotation curves fo galaxies, which doesn't match the expectations if we account only the visible ("normal") matter.\n\nCandidates for being the components of dark matter are beyond the Standard Model of Particles, going for supersymmetry to more exotic stuff like branes and extra dimmensions. It is a very interesting fields and many projects are currently being carried pout to discover it directly or know more about its nature indirectly.\n\nThe remaininf 70% of the Universe is even a bigger mistery, it is called "dark energy". It differs of dark amtter because contrary to it, it doesn't produce an atraction like the gravitational force, but a repulsion in the large scale Universe, so it is not the same as dark matter, and of course, not the same as normal matter, it is something else, there are theories that try to describe it, but we are still in the beggining of discovering what it is.
1012180
Personally I think the reason that there is less antimatter than matter has something to do with the arrow of time.  The last time I checked there was still not a very good explaination of why there is more matter than antimatter.  Its still an outstanding problem in physics.  Its a subject that, as a physicist, I'm very interested in.  Who knows, maybe you'll come up with an explaination.
5763
"open" and "closed" systems usually refer to the exchange of something, usually energy or matter, between a system and its external environment.\n\nfor example, some hot liquid in a tightly-closed thermos bottle could be considered a closed system since it (approximately) cannot exchange matter or thermal energy with the outside world.\n\nit all depends on how you define "universe".  if you define it to mean absolutely everything that exists, then it's reasonable to treat it as a closed system, since there is nothing else for it to exchange something with!\n\nif you define the universe as "the stuff we can see" then it's possible that there is something Way Out There that nobody's detected and that is interacting with the visible universe.\n\none bit of evidence for this might be the expansion of the universe, which is measured via the Doppler red-shift of various galaxies around us.  according to general relativity and the details of the big bang theory, the universe (defined as above - that which we can see) should be expanding at a much greater rate than it is, if it is a closed system.\n\nthere are several postulated explanations for this - one that has gained recent press is the theory of "dark matter" which, in its simplest sense, means "stuff we cannot see that is between us and what we can see".  to date there is no hard evidence for the existence of dark matter.\n\nanother possible explanation is that the universe - that which we can see - is not closed and is interacting with some external agent.  this is perfectly reasonable, as our telescopes have practical limitations and cannot see off into eternity.  so, this does not constitute hard evidence for an "open system" model of the universe, but it remains an open question.
1166578
There is no such thing as negative matter.\n\nYou may be referring to 1 or 3 things:\n\nAnti-matter: Anti-matter is the "anti" particle to various particles. You have the antiproton, antineutron, positron, etc etc. They are basically particles with opposite charges but similiar other characteristics. \n\nExotic matter: Exotic matter is a hypthetical class of matter that violates current particle physics. Such matter hypothetically can have negative mass, have negative kinetic energies while in motion, or even be repelled by gravity. Some 'wormhole' theories rely on such particles.\n\nDark matter: Dark matter is matter of unknown properties that is being theorized to solve many problems associated with our current understanding of the universe. Such mass does not emit or re-absorb light.
326622
When astronomers measure the orbits of stars in galaxies, they are found to be orbiting at a speed that requires more gravity than can be accounted for by all the visible stars and interstellar gas and dust in the galaxy. The assumption is that there is more matter that we can't see with any telescope that is causing the extra gravity. They call it dark matter for lack of a better name.
477879
Well, the one is matter and the other is energy.\n\n=====\n\nIn cosmology, dark matter refers to hypothetical matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. The dark matter hypothesis aims to explain several anomalous astronomical observations, such as anomalies in the rotational speed of galaxies (the galaxy rotation problem). Estimates of the amount of matter present in galaxies, based on gravitational effects, consistently suggest that there is far more matter than is directly observable. The existence of dark matter would also resolve a number of inconsistencies in the Big Bang theory, and is crucial for structure formation.\n\nIf dark matter does exist, it has vastly more mass than the "visible" part of the universe [1]. Only about 4% of the total mass in the universe (as inferred from gravitational effects) can be seen directly. About 22% is thought to be composed of dark matter. The remaining 74% is thought to consist of dark energy, an even stranger component, distributed diffusely in space, that probably cannot be thought of as ordinary particles. Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics. Its urgency is underlined by David B. Cline in a 2003 article in Scientific American, in which he writes: "The terms . . . 'dark matter' and 'dark energy,' serve mainly as expressions of our ignorance." [2]\n\n=====\n\nIn cosmology, dark energy is a hypothetical form of energy which permeates all of space and has strong negative pressure. According to the theory of relativity, the effect of such a negative pressure is qualitatively similar to a force acting in opposition to gravity at large scales. Invoking such an effect is currently the most popular method for explaining recent observations that the universe appears to be expanding at an accelerating rate, as well as accounting for a significant portion of the missing mass in the universe.\n\nTwo proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and quintessence, a dynamic field whose energy density can vary in time and space. Distinguishing between the alternatives requires high-precision measurements of the expansion of the universe to understand how the speed of the expansion changes over time. The rate of expansion is parameterized by the cosmological equation of state. Measuring the equation of state of dark energy is one of the biggest efforts in observational cosmology today.\n\nAdding a cosmological constant to the standard theory of cosmology (i.e. the FLRW metric) has led to a model for cosmology known as the Lambda-CDM model. This model agrees closely with established cosmological observations.\n\nThe term dark energy was coined by Michael Turner.\n\n=====\n\nZack,\n\nDark matter is not detectable.  It can't be seen with visible light, or astronomers would see fuzzy halos around galaxies.\n\nDark energy is a force that is causing the expansion of the Universe to accelerate.  Instead of holding the Universe together, dark energy is blowing it apart.
1153702
Dark energy comprises 70% of the universe and dark matter comprises 26% of the universe. Ordinary baryonic matter, in the form of gas and stars, only makes up 4%.\n\nDark matter refers to matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence may be inferred from gravitational effects on visible matter such as stars and galaxies. Dark matter is explained as being cold, non-baryonic, collision-less dust.\n\nDark energy is a hypothetical form of energy which permeates all of space and has strong negative pressure. According to the theory of relativity, the effect of such a negative pressure is qualitatively similar to a force acting in opposition to gravity at large scales. In other words, dark energy acts as a repulsive force or anti-gravitation and is responsible for the acceleration of the universe.
428553
Good news.  Dark matter is now offered in McDonald's Happy Meals.  You may have to buy more than one, because they're also copromoting with items such as the "Ball O' Quarks" and "Junior Neutron Home Particle Accelerator".
919735
In physics, matter is commonly defined as the substance of which physical objects are composed, not counting the contribution of various energy or force-fields, which are not usually considered to be matter per se (though they may contribute to the mass of objects). Matter constitutes much of the observable universe, although again, light is not ordinarily considered matter. Unfortunately, for scientific purposes, "matter" is somewhat loosely defined.\n\nSee http://en.wikipedia.org/wiki/Matter for more information on matter.\n\nGood Luck!!!
882919
Matter is something that has mass. Mass, according to the general theory of relativity, is caused by the warping of space. A particle that warps space is said to have mass. These particles are known as matter. The terminology is kind of redundant.\n\nNo one knows what the actual particles are made of. Wheeler tried to explain everything as being a warping or curvature of space. But failed.\n\nThe smallest units of matter, that we know of, are leptons (such as electrons) and quarks.