what is dark mattaer regarding the universe? Why? do the galaxies revolve around each other?
101972
Dark matter is just stuff that doesn't give off any radiation. We see stars in space because they're glowing. Planets are considered dark matter because they're just lumps of rocks in space. Far away planets are almost impossible to see even with the strongest telescopes because they don't give off any light. We see them only when they cross in front of a star.\n\nGalaxies revolve around each other for the same reason why planets revolve around the sun, gravity. The gravity of a  galaxy is just on a larger scale, but it's the same principle.
241767
In 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.
362021
We can only give a rough guess as to how many galaxies there are, and it's billions at least. All but the closest ones visible as galaxies to the naked eye have names. Our local galaxy is the Milky Way, which you can see in a dark place on a clear night as a streak of stars running across the sky. A cluster is a group of galaxies.\n\nSome galaxies we have discovered with Hubble and other new telescopes seem to crashing into each other, and the Milky Way may be on its way to having a galactic interloper in the far future.
749553
The Universe is expanding, but individual galaxies are not expanding.  So the stars within a galaxy aren't getting farther apart from each other.  Similarly, our solar system is not expanding (many people get confused about all that).\n\nIn fact, even galaxies within a cluster of galaxies aren't moving apart from each other - for example, the Milky Way galaxy and the Andromeda galaxy are moving *towards* each other, and will collide in a few billion years.  It's the clusters of galaxies that are moving away from each other.\n\nEven though the galaxy itself isn't expanding, the stars are all moving relative to each other.  They all rotate the center of the galaxy, and they also have random motions.  So (as other people above have said) the constellations as we see them today will change - their shapes will become more and more distorted over time.  Here's a neat website which shows what the Big Dipper will look like in the future: http://www.fourmilab.ch/yoursky/help/proper.html
742823
Astonomers look at: (1) how fast galaxies are rotating, (2) how galaxies are moving relative to each other in clusters, and (3) how galaxies focus light from object beyond them ("gravitational lensing"). They then calculate how much mass there must be in those galaxies to explain those phenomena. Then they measure how much light is coming out of those galaxies and use that to estimate how much mass is in the galaxies. \n\nThe two techniques give very different numbers for the mass in the galaxies. The light measurement method yields a much smaller answer for the masses. Therefore, there is matter in the galaxies that is emitting little or no light.\n\nWhat dark matter is made of is a matter for speculation. Some of it may be neutrinos, which are abundant subatomic particles that were previous thought to have no mass but were recently determined to have a little mass. They are very, very numerous. Some may be "exotic" matter that we yet know nothing about.\n\nIt may be more mundane, however. We know that in our neighbor of the Milky Way, most of the starlight comes from just a handful of very bright stars, while most of the mass is in a large number of small, very dim stars.
8989
It looks the secret of the universe is simple.. GRAVITY. \nNothing can escape it not even light. And that gravity is holding the string of the universe together. If the gravity is lost, survival of galaxies would become a question.
969129
Pierre Méchain\n\nIrregular galaxies are those which do not match the standard Hubble scheme of elliptical, spiral and lenticular galaxies. \nThere's only one irregular galaxy in Messier's catalog: M82, and this one is a distorted disk galaxy. The distortion is due to gravitational interaction with its larger and more massive neighbor M81. \n\nAnother, very similar example is at least mentioned in Messier's catalog, though it did not get a number on its own: NGC 5195, the companion of the Whirlpool galaxy M51, which had been discovered by Pierre Méchain. The only further irregular galaxies known to 1782 are the Large and the Small Magellanic Cloud; these are probably miniature disk galaxies in mutual gravitational interaction, and in interaction with our Milky Way.
538835
It's known that dark matter existed before 'something' caused the explosion of light.\n\nSome scientist think that is was a build up of ions/neutrons or some such, which caused the 'bang'.\n\nMany also believe that, because the universe is expanding at a faster rate than previously imagined, some day the whole thing may return to a 'dark' state as galaxies travel beyond the reach of observation.\n\n(By that time, our Galaxy will have been swallowed by the black hole which is keeping it 'captured'.)
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.
286606
The large scale structure of the universe is very interesting! Beyond our own galaxy, there's space filled with varying concentrations of cosmic dust (at an average temperature of 3K = -454.27F), energetic particles, and of course, stars bunched into clusters and galaxies. But the major features of "deep space"  are galaxies and large tracts of "empty" space. \n\nFrom what redshift surveys tell us, galaxies are not distributed uniformly through the universe. Instead, they bunch up together on a large scale (~100Mpc) in structures resembling clusters, bubbles, sheets and filaments. \n1 Mpc = ~3.26 light years, M = 1 million.
665147
only a few galaxies have names: the sombrero, the milky way, andromeda, the black eye, the pinwheel, the small and large malgentic clouds, the whirlpool, and the spindle are all i know of.\n\nMost go by NGC #### in the new star catalouges.\n\nand we only have found a few galaxies.  There are still almost unlimited more!
531320
They really don't. Galaxies come in all sorts of shapes and sizes, spiral, elliptical, round, etc. Few are rotating though, and some in different directions. All seem to be moving away from each other, though, as most scientists agree that the Universe is still expanding,thus blowing a few holes in the old "big bang" theories.
252206
I believe the correct term that you are looking for is "dark matter".\nConsider this: If all of space is filled with very bright stars, then why is it mainly dark.\nScientists came up with the theory of "dark matter", theoretical matter that absorbs the brightness of the stars.\nHowever, my theory is that space is just so completely vast and the stars are so far apart that there is just not enough light to fill it.\nConsider this: you are standing in the middle of a large room with no light. You take out a flashlight...how far can you see? It depends on the size of the room, right. If it is a small room, then the light beam from your flashlight may be able to reach one wall of the room...but say this room is very very large. Would you be able to see the wall then, considering you are standing in the middle of the room?\nSpace has no walls or boundaries...how far would your flashlight see then?...\nSo that's the "dark matter" theory and my theory.\nCiao!
950677
Small "dwarf spheroidal" galaxies tend to be the least dense.  They are hard to detect because they have low surface brightness and barely stand out against the residual sky glow (even from orbit).  They are the least dense because they have survived from the earliest era of small galaxy formation, and have not merged with any other galaxies.\n\nThere are stars in intergalactic space, not associated with any galaxy, that have been flung out of galaxies during collisions.  These are darn near impossible to detect.\n\nLow density galaxies are so hard to detect, and probably merge continuously into a population of stars not in galaxies, that to pick one out as the lowest density would pretty much be a matter of definition, i.e. how small can a galaxy be and still be called a galaxy; does it need to be bound, etc.  \n\nI can forsee some future meeting of the IAU attacked by the public over this issue like the recent furor over Pluto.\n\nGalaxies are really accumulations of Dark Matter first, and have gas and stars as a secondary phenomenon.  It is speculated that there may be many more Dark Matter accumulations, that is to say galaxies with essentially no stars in them, than there are galaxies.
275416
The universe is said to be expanding,at least in the sense\nthat all of the galaxies are supposed to be drifting farther\naway from each other.That would seem to contradict your notion of a tremendously big circle as there's no gravitational pull binding them together.\n Also,if space is finite...then what's outside the boundaries?
752622
Dark matter is matter (the substances which make up physical objects) that does not emit or reflect enough electromagnetic radiation to be directly dected, but its prescence can be assumed by gravitation effects from surrounding physical objects. The composition of dark matter is currently unknown, but current evidence suggests that it is primarily made up of new elementary particles which are collectively being called nonbaryonic dark matter.\nDark matter is though to make up 25% of the universe. \n\nDark energy is a hypothetical form of energy that flows throughout space containing larges amounts of negative pressure and is used to account for the significant amount of missing mass in the known universe. Very little is known about dark energy but it is thought to make up 70% of the universe.
1161939
This is difficult to answer.  Is there a galaxy destroyed everytime I clean my fingernails?\n\nI don't feel we have a good measure of what a galaxy is, our galaxy seems huge, yet we know very little about the boundaries (i.e. what if there were another galaxy right next to ours that we could not reach, like we were different slices of bread, intimately close, yet forever separated)