In astrophysics and cosmology, dark matter is matter of unknown
composition 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.
The dark matter component has vastly more mass than the "visible"
component of the universe. At present, the density of ordinary
baryons and radiation in the universe is estimated to be equivalent
to about one hydrogen atom per cubic metre 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 (Wikipedia, 2007).
VIDEO: "Hubble-Cast: Dark Matter Found"
most (if not all) scientists agree that Dark Matter does indeed,
exists, no one can seem to determine what, exactly it is, or where
it came from.