  In recent years, unprecedented progress in observational cosmology has
revealed a great deal of information about the formation and evolution of
structures in the universe. This, in turn, has raised many challenging issues
for the theorists. In the thesis, we have addressed two such key issues,
namely, (a) the formation of baryonic structures and (b) the nature of dark
matter and dark energy and the limitations in determining their nature from
observations. The main results from the thesis are: (i) The baryons in the
intergalactic medium at redshifts z \sim 2.5 can be modelled (both analytically
and semi-analytically) by accounting for the non-linearities in the density
field through lognormal approximation. Our results agree with observations, and
can be used for constraining parameters related to the baryons. (ii) A simple
model based on baryon conservation, along with observational estimates of
cosmic star formation rate, correctly predicts the abundance of damped
Lyman-alpha systems in the universe. (iii) The redshift distribution of gamma
ray bursts can, in principle, be used for studying the physical conditions of
the universe before reionization epoch (which is otherwise a difficult task).
(iv) It might be possible that the dark matter and dark energy arise from the
same scalar field, provided the equation of state has a dependence on the
length scale. The possibility of using the kinematical and geometrical
measurements (such as supernova observations) for determining the nature and
evolution of the dark energy is discussed.
