Dark energy is basically a kind of energy that exists in the empty spaces in the universe. this phenomenon is not much explored though intriguing theories exist about its presence yet empirical evidence is lacking and inviting the researchers for exploration.
Dear Yasaman ~ we easily sense solid things; and we move around in space that doesn't seem to have much matter in it. But we also know that so-called empty space is permeated with the waves of the Universe. Up till recently, we thought that the Universe was made up only of stars; and that the missing mass was dark matter that we could not see. Now the theory is that all that "emptiness" is actually Our Universe and it is 10 times "heavier" than all the mass that was created out of the Big Bang.