First evidence for Dark Matter has been obtained from the kinematics of galaxies as revealed by ground-based optical observations in the first third of the 20th century. Both in the context of stars within galaxies as well as in the context of galaxies within galaxy clusters, the kinematics of individual objects exceeded the expected velocities inferred from the luminous matter content alone. This effect is so pronounced that it even conflicts with the kinematics needed for gravitational stability of these bound systems. As the luminous matter could not account for this surplus gravitational effect, the term Dark Matter was coined. Since then our understanding has evolved and currently a new particle candidate not accounted for in the standard model of particle physics is the favoured theory to explain Dark Matter. A major observation supporting this paradigm is the multi-wavelength observations of colliding galaxy clusters, where luminous and Dark Matter components are inferred from a measurement of the x-ray gas temperature and via gravitational lensing of background galaxies [Bullet cluster]. The kinematically and spatially different behaviour of the gravitational centres (Dark Matter) with respect to baryonic matter (hydrogen gas) can be very well explained by particle Dark Matter. While Dark Matter seems to pass dissipationless through each other, baryonic matter is heated and is stripped from both galaxy clusters of the collision. Dark Matter can thus well explain the spatially separated gravitational centres observed after the collision, while the major baryonic mass component is still clustered and serenaded from these centres. Since galaxy clusters are typical large scale objects of our universe, Dark Matter is of utmost importance for cosmology and structure formation in the universe. In fact, particle Dark Matter has become a keystone of the standard model of cosmology. [HAP 26.05.2015]
Energy content of the universe
Observations of the last few years led to unified framework referred to as the concordance model. Within this framework, only 4% of the Universe is made of ordinary matter! Following the latest measurements and cosmological models, 73% of the cosmic energy budget seems to consist of Dark Energy and 23% of Dark Matter. Dark Matter turns out to be the main component of cosmic matter. It holds the Universe together through the gravitational force but neither emits nor absorbs light.
The prevalent view is that Dark Matter (DM) consists of stable relic particles from the Big Bang, and that nearly all of it is in the form of Cold Dark Matter (CDM). In the early Universe, CDM particles typically would have already cooled to non-relativistic velocities when decoupling from the expanding and cooling Universe. Hot Dark Matter (HDM) has been relativistic at the time of decoupling. Neutrinos are typical HDM particles; their contribution to the total matter budget is 0.1% - 2%.
Dark Matter has likely played a central role in the formation of large scale structures in the Universe. Its exact nature has yet to be determined. The discovery of new types of particles which may comprise the Dark Matter would confirm a key element of the Universe as we understand it today. The favoured candidate is a particle which is weakly interacting, similarly to neutrinos, but much heavier than the proton: a WIMP (Weakly Interacting Massive Particle). Such a particle is also suggested by SUper-SYmmetric (SUSY) theories of particle physics. [HAP 26.05.2015]
Dark Matter detection
Direct methods look for signals from WIMP interactions with normal matter, e.g. WIMP-nucleus scattering. Since Dark Matter is expected to interact weakly (rarely) and to leave feeble signals, its particles may be first observed in deep underground laboratories, well shielded against cosmic rays and ambient radioactivity which may mimic Dark Matter signals. In order to further progress, experiments are constantly improving their setups to lower detection thresholds and are employing cleaner radiopure materials in detector construction as well as dual signal readout for particle identification. [HAP 26.05.2015]