What is dark energy? What is it composed of?
Good question. As far as I understand it, they don't know yet.
Dark energy is a word used to explain a cosmological constant in cosmology which means that the energy density of the universe is constant, although all the galaxies are moving apart.
So you would initially think that the energy density should be decreasing, but due to the dark energy it actually stays the same.
A good discussion here: http://hubblesite.org/hubble_discoveries/dark_energy/
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