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.