So, dark energy is defined as a hypothetical form of energy composed of two components.  Since theorists have brought back the cosmological constant, one component is the static energy density used to define it. The second component is a dynamic field called quintessence; this has a repulsive nature and promotes the expansion of the universe.  So, when we model the universe with dark matter and dark energy included, we see matter clumping together in a way that is similar to what we observe.  Galaxies will tend to form where the higher matter concentrations are, unfortunately the model tends to produce a greater number of small galaxies than what we observe, which is where dark galaxies come in.