The basic reason he added the cosmological constant was to achieve a static universe. The CC would counteract gravity and keep it from pulling everything in the universe back together, this was sort of a balance between the mass and the gravity in the universe. This idea turned out to be wrong. So, he was wrong to put it there to make the universe static.
Since dark energy is something which is the causing expansion of the universe by counteracting the effects of gravity, it's essentially serving as a CC. Instead of being at equilibrium, dark energy is counteracting gravity so much that it's actually accelerating the expansion of the universe. Einstein didn't put the CC because of dark energy, but dark energy can be stated as CC. This was just a lucky accident, and some of the most remarkable discoveries in science are made by accident, this is one of them.
What a man!!!
I’m not talking about you for once J (I will save my compliments for you in the PM that I am about to send you later), but Einstein… what a man he was. We were so, so lucky that he was behaved well enough in his previous life, to come down to us in human form. He really was one in a trillion. It is hard to imagine how anyone else could ever have come along, and given us the insights and advancements (bombs excepted) that came from that divine brain of his.
It seems that even when he was stumbling around in the dark, or even when he was fiddling his equations to fit perceived notions, he was hitting jackpots. What a guy.
Anyways, let me get this straight:
1) He introduced CC because it would explain why the universe was static.
2) The universe was shown to be expanding, so he took out CC.
3) His original equations now fitted perfectly again with observed phenomena
4) The universe was shown to be expanding faster than anticipated recently, due to Dark Energy
5) Hang on, putting CC back into the equations explains this additional expansion perfectly
That is so fucking bizarre. He really was God.