Abstract
Recent numerous observational data obtained from such independent
sources as angular anisotropies of the cosmic microwave background
radiation, large-scale gravitational clustering of galaxies and their
clusters, observations of supernovae explosions at high redshifts and
the primordial abundance of light elements (combined with the theory
of the primordial nucleosynthesis) prove convincingly that about 95%
of the total energy density of matter in the present Universe is due
to two new and different kinds of matter that have not been
discovered in laboratories and are not contained in the present
standard model of strong, electromagnetic and weak interactions.
The first of them is non-relativistic, non-baryonic dark matter
which is gravitationally clustered similar to the baryonic matter.
The total amount of non-relativistic matter constitutes about 30%
of the present critical density in the Universe. The second of them
is dark energy which is relativistic, gravitationally unclustered
and has negative pressure which modulus is very close to its energy
density. It constitutes the remaining 70% of the critical density.

I discuss how it is possible to derive ("reconstruct") physical
properties of dark energy and dark matter from observations. Different
models of dark matter and dark energy are discussed, with the emphasis
on those most directly connected with the M- and string theory,
in particular, the tachyon matter arising from decay of unstable
D-branes and the Chaplygin gas model which is actually a particular
case of the former model. It is shown that the tachyon model of dark
matter has a generic theoretical problem connected with caustics
formation. The Chaplygin gas model in its original form does not fit
observational data. However, its generalizations are still
investigated.