Dark Matter
Roughly 80 percent of the mass of the universe is made up of material that scientists cannot directly observe. It turns out that roughly 68% of the Universe is dark energy. Dark matter makes up about 27%. The
rest - everything on Earth, everything ever observed with all of our
instruments, all normal matter - adds up to less than 5% of the
Universe. We are much more certain what dark matter is not than we are what it is.
First, it is dark, meaning that it is not in the form of stars and
planets that we see. Observations show that there is far too little
visible matter in the Universe to make up the 27% required by the
observations. Second, it is not in the form of dark clouds of normal
matter, matter made up of particles called baryons. Third, dark matter is not antimatter, because we do not see the unique
gamma rays that are produced when antimatter annihilates with matter.
Finally, we can rule out large galaxy-sized black holes on the basis of
how many gravitational lenses we see. High concentrations of matter bend
light passing near them from objects further away, but we do not see
enough lensing events to suggest that such objects to make up the
required 25% dark matter contribution.
Unlike normal matter, dark matter does not interact with the
electromagnetic force. This means it does not absorb, reflect or emit
light, making it extremely hard to spot. In fact, researchers have been
able to infer the existence of dark matter only from the gravitational
effect it seems to have on visible matter. Many theories say the dark matter particles would be light enough to be
produced at the LHC. If they were created at the LHC, they would escape
through the detectors unnoticed. However, they would carry away energy
and momentum, so physicists could infer their existence from the amount
of energy and momentum “missing” after a collision. Dark matter
candidates arise frequently in theories that suggest physics beyond the
Standard Model, such as supersymmetry and extra dimensions. One theory
suggests the existence of a “Hidden Valley”, a parallel world made of
dark matter having very little in common with matter we know. If one of
these theories proved to be true, it could help scientists gain a better
understanding of the composition of our universe and, in particular,
how galaxies hold together.
Large Scale Structures
The Universe exhibits structure over a wide range of physical scales – from satellites in orbit around a planet through to the galaxy super clusters, galactic sheets, filaments and voids that span significant fractions of the observable Universe. These latter are commonly referred to as the ‘large-scale structure’ of the Universe. In the local Universe, there are two large-scale structures of
particular importance: the Great Wall and the Great Attractor. These
structures influence the motions of galaxies in the Local Group, and are ultimately responsible for the fate of the Milky Way. In physical cosmology, the term large-scale structure refers to the
characterization of observable distributions of matter and light on the
largest scales (typically on the order of billions of light-years).
Prior to 1989, it was commonly assumed that galaxy
clusters were the largest structures in existence, and that they were
distributed more or less uniformly throughout the universe in every
direction. However, based on redshift survey data, in 1989
Margaret Geller and John Huchra discovered the "Great Wall," a sheet of
galaxies more than 500 million light-years long and 200 million wide,
but only 15 million light-years thick.
References:
http://www.sciencedaily.com/articles/l/large-scale_structure_of_the_cosmos.htm
http://astronomy.swin.edu.au/cosmos/L/large-scale+structure
http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/
http://astro.berkeley.edu/~mwhite/darkmatter/dm.html
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