Dark Matter

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Evidence for Dark Matter

Today, we have striking astrophysical evidence that Dark Matter really exists. This is matter that is not made out of the particles described by the Standard Model of Particle Physics. It does not emit any kind of radiation (therefore we cannot "see" it), and probably interacts only via the weak interaction. However, it has a finite mass and can be detected due to the associated gravitational force.

Some of the evidences for Dark Matter are:

  • The rotation curves of galaxies.
  • The dynamics of galaxy clusters.
  • The mass of galaxy clusters that act as gravitational lenses is much larger than the visible (in the whole spectral range) amount of stars, gas, and dust.
  • A Fourier analysis of the fluctuations in the cosmic microwave background (CMB) measured with WMAP allows to determine fundamental cosmological parameters very precisely.

The Cosmic Recipe

According to the analysis of the WMAP data, our universe is made up of
  • 4% ordinary (baryonic) matter,
  • 22% cold Dark Matter that builds large structures in the Universe,
  • and 74% of Dark Energy, the energy content of the empty space. This mysterious Dark Energy is responsible for the accelerated expansion of the Universe.
This facts can be summarized in the following way:
Today we know that we only know about 5% of the Universe.
Figure courtesy of WMAP Figure courtesy of WMAP

What is Dark Matter?

The only thing we know for sure about Dark Matter is that it really exists, that it builds gravitationally bound clumps, and that is is a key ingredient to understand the evolution of the large scale cosmic structures. It has to be made of neutral, massive particles that possibly interact only via weak interactions. These weakly interacting massive particles (WIMPs) have not been discovered yet, but many experiments around the globe are trying to find the WIMP either directly (like the XENON experiment), or indirectly by searching for rare WIMP annihilation processes in the galaxy (Fermi/GLAST).

It is also possible that Dark Matter will be created in the high energy collisions of the LHC accelerator at CERN. This idea is mainly supported by supersymmetric extensions of the Standard Model (SUSY) that predict many new particles. The lightest SUSY particle (in most theories the so-called neutralino) should be stable and is therefore a strong candidate to be the Dark Matter particle.

A collection of review papers on Dark Matter can be found here.

updated: 10/16/2007 by Marc Schumann