As with so much of the most ancient history of the Imperium, the origin of the Dark Angels is lost in legend. All that is known for sure of the early history of the Chapter is that they were the first and the largest of the original Space Marine Legions in what has become known as the Great Crusade. During these times their Primarch, Lion 'El Jonson, was instrumental in planning some of the most difficult campaigns. Following the terrible blow of the Warmaster's treason, the history becomes confused and obscure. It is thought that Jonson survived the Heresy but of his fate immediately after, nothing is known for sure - although rumor and speculation are rife. Some sources have it that Jonson was killed in battle, with a monster of the warp, on his way back to the Dark Angels’ home world of Caliban. It seems more likely however that he was caught up in the still unexplained destruction of Caliban itself. A recurring legend has it that Jonson still roams the Galaxy, searching for some unknown enemy. This enemy may be alluded too obliquely in the annals of the Dark Angels themselves.
After the Great Betrayal, the Dark Angels were recognized into a Chapter, in line with the other Space Marine Legions that remained, and have since sired several Successor Chapters. Of these, the Angels of Absolution, the Angels of Redemptions and the Angels of Vengeance have particularly close links with the Dark Angels themselves. The four Chapters collectively call themselves the Unforgiven, although a satisfactory answer is yet to be given as to why this should be. In the last ten millennia, the Dark Angels have continued their Primarch’s tradition of conducting particularly effective and well-planned campaigns. Although, some observers have noted instances where the Chapter, or one of its successors, have failed to support Imperial forces or take other action when requested to do so, and have further failed to give a satisfactory explanations for this. In at least seven hundred and fifty reliably documented instances, the 'Unforgiven' has abandoned a campaign at a critical juncture with no reason being offered to explain this extraordinary behavior. It is postulated that these Chapters may have some hidden agenda, which has priority even over service to the Emperor himself. This is, to say the least, possibly treasonable and at worst Heresy. To date, however, despite continued and intensive covert inquisitorial investigation, the Adeptus Terra is still unable to explain this anomalous behavior in an otherwise superb service record.
I am making use of the Space Marine revision from the Space Marine Ideas page.
And here are the army lists for the Dark Angels Space Marine Chapter: