It's dark out there...
"Dark matter"
refers to matter of an
unknown nature that many astronomers
and cosmologists
think must make up the majority of the mass in the universe. Its presence is revealed by the
gravitational effects on
objects that we can see. According to the current understanding of how
gravity works, the way the visible matter behaves indicates that there
should be much more matter than we can detect — and therefore,
much more mass exerting a gravitational influence — in objects in
space, like stars in galaxies, or
galaxies in clusters.
The clusters move at
speeds that are too high to be attributed just to the visible
galaxies.
If we were to apply the rules of gravity to the matter that we can
see, galaxies (and galaxy clusters) would fly apart, losing the
swiftly moving outer components, because there isn't enough mass (and
therefore gravity) present to hold them in place. So, if more mass is
added to the visible matter, the equations work, objects remain in
their paths, and everything makes sense, mathematically. Results
from the
Wilkinson Microwave Anisotropy Probe (WMAP) show that roughy 1/4 of the
mass of the universe is composed of dark matter. But what is it,
really?
There are many theories as to what comprises dark matter. It is
unlikely to be any one substance, but rather a variety of substances
that contribute to the total needed to hold things together. Some
aspects are fairly certain: the objects would not give off much,
if any, visible light, and so may include black holes, brown dwarfs,
neutron stars, red dwarfs, and planets;
they may also be very small individually, distributed somewhat equally
in a very large volume, like a cloud. That could include particles such
as axions, neutrinos and neutralinos, as well as other particles, both
exotic and commonplace. However, there are still many missing pieces to
this puzzle, and scientists continue to search for more pieces.
While the individual components are difficult to find, the influence of
dark matter is easily detected, and comparatively simple to measure.
Astronomers measure high temperature gas in
these galaxy clusters. This gas is at too high a temperature to
remain bound to the cluster without some additional mass, hidden from
view. For galaxies and groups, the X-ray data have often indicated very
extended dark matter halos far beyond the radius at which one sees
starlight or galaxies. The total inferred dark matter mass is often
several times that in the "visible" galaxies alone. Additionally, a
phenomenon called gravitational
lensing acts as a more visually obvious way to demonstrate how a
galaxy's mass (and therefore gravity) can bend the rays of light
traveling from a distant object to Earth.
For the time being, dark matter can be thought of as a way to fill in a
variable in an equation — the definite quantity of indefinite
substance, or the indefinite influence of some as-yet undetectable
force. As some scientists explore the possibilities of known-substance
candidates for dark matter, others are also searching for new
"supersymmetric" particles, i.e., hypothesized particles that are
partners to the particles that are already known, as with the research
being done by the European Organization for Nuclear Research, or CERN, with
the Large
Hadron Collider. Also, some cosmologists hypothesize that gravity
from our dimension could be exerting influence on matter in other
dimensions, and perhaps vice-versa, which is also among the LHC
research objectives.
Last Updated: February 2011
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