What is Dark Matter?

dark matter is a hypothetical form of matter that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter. According to present observations of structures larger than galaxies, as well as Big Bang cosmology, dark matter accounts for the vast majority of mass in the observable universe. The observed phenomena consistent with dark matter observations include the rotational speeds of galaxies, orbital velocities of galaxies in clusters, gravitational lensing of background objects by galaxy clusters such as the Bullet cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark matter also plays a central role in structure formation and galaxy evolution, and has measurable effects on the anisotropy of the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation: the remainder is called the "dark matter component".The composition of dark matter is unknown, but may include ordinary and heavy neutrinos, recently postulated elementary particles such as WIMPs and axions, astronomical bodies such as dwarf stars and planets (collectively called MACHOs), primordial black holes and clouds of nonluminous gas.

I like Laurahals answer. As far as I can tell it exists only because Galaxies don't behave the way our theories say they should behave. To make our observations match our theories the concept of dark matter was invented. Science isn't supposed to work like that. The closest analogy I can think of was during the middle ages observations of planetary orbits did not match theoretical circular orbits. Obviously the observations HAD to be wrong, it couldn't be the theories. So astronomers developed epicycles: circular orbits on top of orbits. They were quite clever and worked fine, until better observations were made. So astronomers did the obvious thing and added epicycles on top of other epicycles. By the time Copernicus came along the system was absurdly complex. Of course when the copernican system didn't work quite right the solution was (drum-roll) more epicycles. Not until Kepler came along did epicycles die a well deserved death. Now its beginning again. Invisible solutions. Dark matter may exist. But its bad science.

Possibilities for dark matter range from tiny subatomic particles weighing 100,000 times less than an electron to black holes with masses millions of times that of the sun (9). The two main categories that scientists consider as possible candidates for dark matter have been dubbed MACHOs (Massive Astrophysical Compact Halo Objects), and WIMPs (Weakly Interacting Massive Particles). Although these acronyms are amusing, they can help you remember which is which. MACHOs are the big, strong dark matter objects ranging in size from small stars to super massive black holes (1). MACHOs are made of 'ordinary' matter, which is called baryonic matter. WIMPs, on the other hand, are the little weak subatomic dark matter candidates, which are thought to be made of stuff other than ordinary matter, called non-baryonic matter. Astronomers search for MACHOs and particle physicists look for WIMPs.

Astronomers and particle physicists disagree about what they think dark matter is. Walter Stockwell, of the dark matter team at the Center for Particle Astrophysics at U.C. Berkeley, describes this difference. "The nature of what we find to be the dark matter will have a great effect on particle physics and astronomy. The controversy starts when people made theories of what this matter could be--and the first split is between ordinary baryonic matter and non-baryonic matter" (10). Since MACHOs are too far away and WIMPs are too small to be seen, astronomers and particle physicists have devised ways of trying to infer their existence.

A Thanksgiving turkey includes both dark matter and white matter. I like the white matter with gravy on it. The dark matter is better in a sandwich.

It's something dark which sometimes matter...mostly by effects....so basically if there are some effects which should be caused by matter, but there is no sign of matter there, well then scientist use to call the "dark" something "dark matter"....by all means everything regarding dark matter is mere speculation.

http://en.wikipedia.org/wiki/Dark_matter

http://www.nasa.gov/multimedia/imagegallery/image_...

A fudge factor to explain some otherwise-anomalous gravitational effects.

Addendum: There is new physics to be discovered here. We just haven't figured out what it is yet. I look forward to seeing what the cosmologists come up with.

think of it this way: it's really nothing. dark matter is on the outer part of our universe.

where nothing exists. nothing can live. and plus: 'it's dark'.

go in the courting maybe it dark for the people who dress in black