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The Universe Darkly
The Universe Darkly
When you look up at the night sky, you see a lot of things glowing like stars, planets, and galaxies. (2008-02-29)

Jelly Bean
The Universe in a Jelly Bean Jar Flash
Most of the Universe is dark. The protons, neutrons and electrons that make up the stars, planets and us represent only a small fraction of the mass and energy of the Universe.

The Universe
The Universe, by Chandra
The two largest pieces of the Universe, dark matter and dark energy, are the two that we know the least about, yet nothing less than the ultimate fate of the Universe will be determined by them.

Dark Matter
A term used to describe matter that can be inferred to exist from its gravitational effects, but does not emit or absorb detectable amounts of light.
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Evidence for Dark Matter
Evidence for Dark Matter

Observations of the rotational speed of spiral galaxies, the confinement of hot gas in galaxies and clusters of galaxies, the random motions of galaxies in clusters, the gravitational lensing of background objects, and the observed fluctuations in the cosmic microwave background radiation require the presence of additional gravity, which can be explained by the existence of dark matter.

Amount of Dark Matter

The evidence suggests that the mass of dark matter in galaxies, clusters of galaxies, and the universe as a whole is about five or six times greater than the mass of ordinary light-emitting matter that makes up stars, planets, gas and dust.

Alternatives to Dark Matter
Alternatives to Dark Matter

One possibility, considered unlikely by most astrophysicists, is that a modification of the theory of gravity can explain the effect attributed to dark matter.

What is Dark Matter?
What is Dark Matter

The nature of dark matter is unknown. A substantial body of evidence indicates that it cannot be baryonic matter, i.e., protons and neutrons. The favored model is that dark matter is mostly composed of exotic particles formed when the universe was a fraction of a second old. Such particles, which would require an extension of the so-called Standard Model of elementary particle physics, could be WIMPs (weakly interacting massive particles), or axions, or sterile neutrinos.

Detection of Dark Matter Particles
Dwarf Galaxy

Various types of experimental searches for dark matter candidates are being pursued by a number of investigators: the direct detection of dark matter particles using innovative new detectors; the detection of X-rays or gamma-rays from the decay or annihilation of dark matter particles; and the detection of dark matter particles created by colliding beams of high energy protons.

This animation illustrates the three major components of a cluster of galaxies:
bright galaxies, hot gas (shown in orange), and the dominant component, dark matter (shown in pink).

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