what is dark matter?

Question

except for what is in a blondes brain, lol. just kidding about that bit.

Answer 1

DARK MATTER
We believe that most of the matter in the universe is dark, i.e. cannot be detected from the light which it emits (or fails to emit). This is "stuff" which cannot be seen directly -- so what makes us think that it exists at all? Its presence is inferred indirectly from the motions of astronomical objects, specifically stellar, galactic, and galaxy cluster/supercluster observations. It is also required in order to enable gravity to amplify the small fluctuations in the Cosmic Microwave Background enough to form the large-scale structures that we see in the universe today.

For each of the stellar, galactic, and galaxy cluster/supercluster observations the basic principle is that if we measure velocities in some region, then there has to be enough mass there for gravity to stop all the objects flying apart. When such velocity measurements are done on large scales, it turns out that the amount of inferred mass is much more than can be explained by the luminous stuff. Hence we infer that there is dark matter in the Universe.
Dark matter has important consequences for the evolution of the Universe and the structure within it. According to general relativity, the Universe must conform to one of three possible types: open, flat, or closed. The total amount of mass and energy in the universe determines which of the three possibilities applies to the Universe. In the case of an open Universe, the total mass and energy density (denoted by the greek letter Omega) is less than unity. If the Universe is closed, Omega is greater than unity. For the case where Omega is exactly equal to one the Universe is "flat".

Note that the dynamics of the Universe are not determined entirely by the geometry (open, closed or flat) unless the Universe contains only matter. In our Universe, where most of Omega comes from dark energy, this relation between the mass density, spatial curvature and the future of the universe no longer holds. It is then no longer true in this case that "geometry (spatial curvature) is destiny." Instead, to find out what will happen one needs to calculate the evolution of the expansion factor of the universe for the specific case of matter density, spatial curvature and "funny energy" to find out what will happen.

Dark matter (DM) candidates are usually split into two broad categories, with the second category being further sub-divided:

Baryonic
Non-Baryonic
hot dark matter (HDM) and
cold dark matter (CDM),
depending on their respective masses and speeds. CDM candidates travel at slow speeds (hence "cold") or have little pressure, while HDM candidates move rapidly (hence "hot").
For more information

read the Essay by Joe Silk,
learn more about the Hubble expansion
see Wikipedia's articles on Physical Cosmology and dark matter
see the NCSA's Cosmos in a Computer site, which has a lot of information on dark matter and cosmology in general, including movies simulating the universe,
look at the Lambda web site
read this popular level article, which outlines the different dark matter candidates,
or see one of the books or articles in the Bibliography.

Aside on flatness
Current indications from the cosmic microwave background are that the universe is spatially flat. That implies that the sum of all of the energy (density) in the universe equals the critical density, i.e. the total Omega is 1. This is quite interesting because as the Universe expands the value of Omega changes. In fact the value 1 is unstable, and the Universe would prefer to evolve towards one of the two natural values: 0, if the expands forever further apart until the Universe is almost totally empty ; and infinity, if the matter recollapses to a state of higher and higher density. Then the observation that Omega is fairly close to 1 today, means that it must have been even closer to 1 in the past. It is unsatisfying to believe that we just happen to live at the time when Omega is just starting to depart from 1 by a small factor. It is much more appealing to consider that we do not live at a special epoch, so that Omega is still close to 1 today. But then we need to explain why Omega started out very close to 1 in the early universe. The theory of inflation provides just such a justification - most versions of inflation predict that the early Universe was driven extremely close to flat, and that it is still very close to flat today. If this is so, then at least 90% the energy of the Universe is dark! Note that although the universe may be flat, that does not mean that matter makes up the critical density. In addition to dark matter there is dark energy, e.g. a cosmological constant, that needs to be included in the accounting.

Answer 2

I agree with Emucompb to a point.
Dark matter was invented by some guy to try to explain some thing that may not exist.
He assumed that a galaxy was a satellite system.
The outer arms turn locked with the center.
If it were a satellite system the outer arms would fly away.
It would happen in our solar system so they must slow down as the orbits get higher.
Without dark matter there is some other mechanism at work the does not require a slower speed or dark matter.

Answer 3

The part of the Universe that we can see -- all the stars, galaxies, planets, etc. -- only accounts for a relatively small percentage of the matter in the Universe. The rest, which is pretty much beyond human ability to detect at this point, is called "dark matter".

Answer 4

The theory goes that the observable universe is too heavy to account for all the matter we know about and therefore there must be some 'invisible' matter that we can't see - dark matter, by calculating the difference in weight we can know just how much dark matter there is.

Answer 5

We don't know what dark matter is made of. At this point, we only know it exists because of the gravity it exerts on stars and galaxies that we can see. There are several guesses on what it could be: WIMPs (weakly interacting massive particles) are the best guess at this point in time, but we have not seen such particles in our accelerators yet, so that is purely speculation.

Answer 6

It's a mathematical fudge factor for the gravity in galaxies that we don't observe from ordinary matter -- galaxies hold together better than observed ordinary matter would suggest, so some bright guy invented "exotic dark matter" to explain where the other gravity is coming from.

Answer 7

it is a theoretical substance which consists approx 90% of all matter in the universe and is totally invisible and undetectable
in other words no one knows but one theory is that it is responsible for the current rate of expansion of the universe and why it wont continue expanding forever

Answer 8

Dark matter is matter, not directly observed and of unknown composition, that does not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence can be inferred from gravitational effects on visible matter

Answer 9

In between Tony Blairs ears

Answer 10

it is, quite simply, all the stuff in the cosmos that we can't see because it doesn't emit any light (only stars do this) and is too small or far away to reflect any. Mostly it's dust, but it would also include very distant planets and their satellites, were present.