Can't it just be planets, asteroids, dust or gas which do not emit enough light to reach us?
No. Dust and gas have been ruled out. (Larger bodies weren't really in the running). Dust and gas will heat up and emit black body radiation. We don't see this.
Whatever it is, it isn't interacting with EM radiation at all.
There are MACHOS and WIMPS that are thought to be dark matter, right? However, I forgot what they stand for, or what they are, exactly.
MACHOS are massive compact halo objects, large bits of rock, black holes, dead stars that are too cool to detect.
I think they are pretty much ruled out as the source of all the dark matter, if there were enough of them we would see them as lensing effects as they pass in front of distant visible objects.
WIMPS are weakly interacting massive particles, some undiscovered particle like a neutrino that has enough mass to make up the dark matter, but no charge so it doesn't have a detecable effect on light.
The only thing known for certain about dark matter is what it is not. Specifically it is NOT made up of baryonic matter (stuff made from neutrons and protons). This rules out rocks, stars, etc.
doesnt that require knowing the rate at which they are moving across the sky? how is that calculated?
I imagine you just observe a large number of distant galaxies and look for short term variations in their brightness. From the rate of transient events you can work out the density of objects in the halo, although probably not anything about the halo object.
so you use the rotation curve of the galaxy. that makes sense.
So let me get this straight Mathman, Dark Matter doesn't interact at all with any charged subatomic particles? What about Neutrinos?
-no wonder why none of our instruments will detect it.
I sure hope CERN turns something up.
What is the relationship between dark matter and dark energy? despite matter is a form of energy....
Not quite - you use distant background galaxies as a set of randomly distributed point sources.
As the nearby dark object goes in front of a galaxy there will be a lensing effect that will dim/brighten the image of the distant galaxy.
From the amount of lensing you can get an estimate of the mass/size of the invisible object.
The theory of the synthesis of elements in the early universe (big bang nucleosynthesis), and the observation of present relative abundances of elements indicate that, as mathman said, dark matter is not made of protons and neutrons (or other baryons).
Dark matter is thought to interact with normal matter only through gravity (observed) and the weak nuclear force (theoretical). Maybe CERN will turn up some of the particles predicted by supersymmetry, and, if this happens, this will give credence to the idea that supersymmetric stuff makes up dark matter.
Neutrinos do contribute, but their contribution is not that significant.
In conventional models, there is no link between dark matter and dark energy.
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