Dark Matter

From the observation of the rotation of galaxies resembling our own Milky Way, astronomers have found that there must be over ten times more mass in them than is visible as light emitting stars. This puzzling discrepancy indicates that there has to exist some matter which is not emitting light and is, therefore, called Dark Matter. This invisible matter makes up more than 90% of the mass of galaxies and also of larger structures and, therefore, of our universe. However, it is still unclear what kind of matter it really is. This leads to one of the presently most challenging and intriguing questions of physics: What does most of our universe consist of?

Dark matter could be made of MACHOs, huge gas balls which are too small to ignite and to shine as stars. Or it could be made of faint stars overlooked by our telescopes. Or it could be made of black holes left over from the big bang. However searches for all these dark matter candidates have failed, re-emphasizing the significance of the direct search for elementary particle dark matter, amongst others undertaken by the CRESST experiment.

A well-motivated candidate for the dark matter particle is the so-called WIMP (Weakly Interacting Massive Particle). Theories suggest that WIMPs are present everywhere in our galaxy and in a vast halo around it. In spite of having a mass similar to that of a nucleus like, e.g., iron, their extremely weak interaction with normal matter enables them to travel easily through the earth or even through our galaxy without a single interaction. Dark matter particles with lower masses are,e.g., suggested by asymmetric dark matter models. In both cases the interaction cross-section between a dark matter particle and the nucleus of a target is extremely low, making the direct search for dark matter one of the most challenging and difficult tasks of modern astroparticle physics.

The Spiral Galaxy M66; taken from the European Southern Observatory


Globular Cluster NGC 6093; taken from the Hubblesite
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