What is dark matter?

Answer 1

In astrophysics, dark matter refers to matter that does not emit or reflect enough electromagnetic radiation (such as light, x-rays and so on) to be detected directly, but whose presence may be inferred from its gravitational effects on visible matter. Among the observed phenomena consistent with the existence of dark matter are the rotational speeds of galaxies and 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 Big Bang nucleosynthesis, 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 is directly observable, indicating that the remainder is dark.

The composition of dark matter is unknown, but may include new elementary particles such as WIMPs and axions, ordinary and heavy neutrinos, dwarf stars and planets collectively called MACHOs, and clouds of nonluminous gas. Current evidence favors models in which the primary component of dark matter is new elementary particles, collectively called nonbaryonic dark matter.

The dark matter component has vastly more mass than the "visible" component of the universe. [1] At present, the density of ordinary baryons and radiation in the universe is estimated to be equivalent to about one hydrogen atom per cubic meter of space. Only about 4% of the total energy density 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. [2] Some hard-to-detect baryonic matter (see baryonic dark matter) makes a contribution to dark matter, but constitutes only a small portion. [3] [4] Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics. It has been noted that dark matter and dark energy serve mainly as expressions of our ignorance, much as the marking of early maps with terra incognita. [2]

Answer 2

It's simply matter that we haven't been able to see with a telescope.

Many possibilities are being considered for exactly what it could be, but no one knows. The possibilities are real, and make some kind of sense, but no one possibility makes more sense than the others.

We believe it exists because the galaxies are otherwise too lightweight to hold together, they should just fly apart. Dark matter exerts gravitational force and holds them together. We can see the effect of dark matter, we just can't see the matter in a telescope.

Answer 3

Numerous candidates for the dark matter component in the halos of galaxies and clusters of galaxies have been proposed over the years, but no successful detection of any of them has yet occurred. If the dark matter is not made of the same material as the nuclei of ordinary atoms, then it may consist of exotic particles capable of interacting with ordinary matter only through the gravitational and weak nuclear forces. The latter property lends these hypothetical particles the generic name WIMPs, after weakly interacting massive particles. Even if WIMPs bombarded each square centimetre of the Earth at a rate of one per second (as they would do if they had, for example, individually 100 times the mass of a proton and collectively enough mass to “close” the universe; see below), they would then still be extremely difficult—though not impossible—to detect experimentally.

Another possibility is that the dark matter is (or was) composed of ordinary matter at a microscopic level but is essentially nonluminous at a meaningful astronomical level. Examples would be brown dwarfs (starlike objects too low in mass to fuse hydrogen in their interiors), dead white dwarfs, neutron stars, and black holes. If the objects are only extremely faint (e.g., brown dwarfs), they can eventually be found by very sensitive searches, perhaps at near-infrared wavelengths. On the other hand, if they emit no light at all, then other strategies will be needed to find them—for example, to search halo stars for evidence of “microlensing” (i.e., the temporary amplification of the brightness of background sources through the gravitational bending of their light rays).

Answer 4

If you can answer that, you will be famous!

Astronomers have noticed that if you add up all of the gravity from all of the visible objects that we can see, then there should not be enough gravity to hold the galaxies together. So the term "dark matter" was invented as a way of explaining where all of the 'extra' gravity was coming from.

Answer 5

Unknown mass responsible for a significant portion of the gravity in the universe...
There is too much gravity around to be explained by the visible matter in the universe so we call what we cannot see "dark matter"

Answer 6

Dark matter is an "invention" of the astro physicists to explain a big deal of gravity obviously existing in the universe but explicable by what we see. Per definitzion it is a kind of matter of which we don't know the characteristics yet (for we can neitehr see nor measure it), but we assume at least that it contains lots of gravity.
(What a stupid silly try of the scientists!!)

Answer 7

dark matter refers to matter that does not emit or reflect enough electromagnetic radiation (such as light, x-rays and so on) to be detected directly, but whose presence may be inferred from its gravitational effects on visible matter.

Answer 8

dark matter is matter that can not be seen by us it does not reflect light, also some people belive that it does not exist

Answer 9

Precisely? No one knows what it's made out of, what it consists of. My guess is antimatter, but I have nothing to back up that theory except it hasn't been proved otherwise.

Answer 10

The unmeasurable glue that holds the galaxies together