Some time ago the 'Dark Energy Task Force' reported:
http://arxiv.org/abs/astro-ph/0609591

"Dark energy appears to be the dominant component of the physical
Universe, yet there is no persuasive theoretical explanation for its
existence or magnitude. The acceleration of the Universe is, along
with dark matter, the observed phenomenon that most directly
demonstrates that our theories of fundamental particles and gravity
are either incorrect or incomplete. Most experts believe that nothing
short of a revolution in our understanding of fundamental physics will
be required to achieve a full understanding of the cosmic
acceleration....."


This issue now seems to be successfully addressed by the following
paper:

Unravelling the Dark Matter - Dark Energy Paradigm
Reginald T Cahill (Flinders University)
http://arxiv.org/abs/0901.4140

Abstract:
The standard LambdaCDM model of cosmology is usually understood to
arise from demanding that the Friedmann-Lemaitre-Robertson-Walker
(FLRW) metric satisfy the General Relativity dynamics for spacetime
metrics. The FLRW data-based dominant parameter values,
Omega_Lambda=0.73 and Omega_m=0.27 for the dark energy and dark
matter+matter, respectively, are then determined by fitting the
supernova red-shift data. However in the pressure-less flat-space case
the LambdaCDM model is most easily derived from Newtonian gravity, and
which was based on the special case of planetary motion in the solar
system. Not surprisingly when extended to galactic rotations and
cosmology Newtonian dynamics is found to be wanting, and the fix-up
involves introducing dark matter and dark energy, as shown herein.

However a different theory of gravity leads to a different account of
galactic rotations and cosmology, and does not require dark matter nor
dark energy to fit the supernova data. It is shown that fitting the
LambdaCDM model to this new model, and so independently of the actual
supernova data, requires the LambdaCDM model parameters to be those
given above. Hence we conclude that dark energy and dark matter are no
more than mathematical artifacts to fix-up limitations of Newtonian
gravity. Various other data are also briefly reviewed to illustrate
other successful tests of this new theory of gravity.