Possibility of making dark energy equivalent with dark matter I was curious whether it is possible to make dark energy equivalent to dark matter. 
Can this unification be done?
If it can, why do scientists prefer to separate dark energy from dark matter?
 A: Despite the names, dark matter and dark energy are not related.  Dark matter is similar to the baryonic matter that makes up galaxies, planets, you, etc.  The key difference is that dark matter doesn't interact via the electromagnetic force, so you can't directly observe it.  Dark matter is most likely a new sub-atomic particle, and it make be a WIMP (weakly-interacting massive particle).  That is, it may interact through the weak force.  One example of dark matter is hot dark matter, the neutrino.  However, the type of dark matter relevant in cosmology is cold dark matter, which hangs around galaxies in very large clumps.  We can indirectly observer dark matter through gravitational lensing, such as in the case of the Bullet Cluster.
Dark energy is quite different.  Dark energy is the popular term for the cosmological constant, a negative pressure vacuum energy density.  Usually, dark energy is modeled as a negative pressure fluid that has an equation of state of w = -1.  This has the effect of accelerating the expansion of the universe.
A: This is exactly what dark fluid hypothesis is about:
http://arxiv.org/abs/astro-ph/0601274
http://arxiv.org/abs/0804.1588
Not all the theories under the guise of the 'dark fluid' brand are the same though, that first paper postulates a scalar, while the other from Zhao assumes a U(1) electromagnetic-like vector field
However, the scientific bottleneck at the moment is not so much the lack of theories as the quality of the observational data, is still very hard to constrain many of these models by observations alone.
