The discoveries of dark matter and the cosmic web are two stages of a typical scientific revolution (Kuhn 1970, Tremaine 1987). As often in a paradigm shift, there was no single discovery, new concepts were developed step-by-step.
First of all, actually there are two dark matter problems - the local dark matter close to the plane of our Galaxy, and the global dark matter surrounding galaxies and clusters of galaxies. Dark matter in the Galactic disk is baryonic (faint stars or jupiters), since a collection of matter close to the galactic plane is possible, if it has formed by contraction of pre-stellar matter towards the plane and dissipation of the extra energy, that has conserved the flat shape of the population. The amount of local dark matter is low, it depends on the mass boundary between luminous stars and faint invisible stars.
The global dark matter is the dominating mass component in the Universe; it is concentrated in galaxies, clusters and superclusters, and populates also cosmic voids. Global dark matter must be non-baryonic, its density fluctuations start to grow much earlier than perturbations in the baryonic matter, and have at the recombination epoch the amplitude large enough to form all structures seen in the Universe. Initially neutrinos were suggested as particles of dark matter (hot dark matter), but presently some other weakly interacting massive particles, called cold dark matter, are preferred.
Recently direct observational evidence was found for the presence of Dark (or vacuum) Energy. New data suggest that the total matter/energy density of the Universe is equal to the critical cosmological density, the density of baryonic matter is about 0.04 of the critical density, the density of dark matter is about 0.23 of the critical density, and the rest is dark energy.
A number of current and future astronomical experiments have the aim to get additional data on the structure and evolution of the Universe and the nature and properties of dark matter and dark energy. Two astronomical space observatories are planned to be launched in 2008: the Planck CMB mission and the Herschel 3.5-m infrared telescope. The main goal of the Planck mission is to measure the CMB radiation with a precision and sensitivity about ten times higher than those of the WMAP satellite. This allows to estimate the values of the cosmological parameters with a very high accuracy. The Herschel telescope covers the spectral range from the far-infrared to sub-millimeter wavelengths and allows to study very distant redshifted objects, i.e young galaxies and clusters.
Very distant galaxies are the target of the joint project GOODS - The Great Observatories Origins Deep Survey. Observations are made at different wavelengths with various telescopes: the Hubble Space Telescope, the Chandra X-ray telescope, the Spitzer infrared space telescope, and by great ground-based telescopes (the 10-m Keck telescope in Hawaii, the 8.2-m ESO VLT-telescopes in Chile). Distant cluster survey is in progress in ESO.
NASA-DOE have approved the mission DESTINY - the Dark Energy Space Telescope. Its goal is to detect and obtain precision photometry, light-curves and redshifts of over 2000 type Ia supernovae over the redshift range 0.5 < z < 1.7 to constrain the nature of dark energy.
The largest so far planned space telescope is The James Webb Space Telescope (JWSP) - a 6.5-m infrared optimized telescope, scheduled for launch in 2013. The main goal is to observe first galaxies that formed in the early Universe.
To investigate the detailed structure of our own Galaxy the space mission GAIA will be launched in 2011. It will measure positions, proper motions, distances and photometric data for 1 billion stars, repeatedly. Its main goal is to clarify the origin and evolution of our Galaxy and to probe the distribution of dark matter within the Galaxy.
The story of the dark matter and dark energy is not over yet - we still do not know of what non-baryonic particles the dark matter is made of, and the nature of dark energy is also unknown. Both problems are a challenge for physics. So far the direct information of both dark components of the Universe comes solely from astronomical observations.
Acknowledgments
The study of dark matter and large-scale structure of the Universe in Tartu Observatory is supported by Estonian Science Foundation grant 6104, and Estonian Ministry for Education and Science grant TO 0060058S98. I thank Astrophysikalisches Institut Potsdam (using DFG-grant 436 EST 17/2/06), and the Aspen Center for Physics for hospitality, where part of this study was performed, and Elmo Tempel and Triin Einasto for help in preparing the bibliography. Fruitful discussions with Enn Saar, Maret Einasto, Alar Toomre, and the editor of the series Prof. Bozena Czerny are acknowledged.
