It used to be a very static headquarters called Aldurukh (Ancient Calibanite for "Rock of Eternity"), back when it was the fortress-monastery on Caliban. Unfortunately Caliban was destroyed after the Dark Angels mercilessly bombarded their own home after Luther roused the entire planet into rebellion against the Lion, taking the Dark Angels garrison force on Caliban with him. After the planet's destruction, the fortress-monastery's reinforced foundation coupled with the fact that its void shields were online meant that it was the largest piece of the planet remaining, so the Dark Angels put engines on it and now they fly around the galaxy, recruiting from tribal populations on the worlds they visit.
It also seems to have a ton of dark tech factories, which would explain the ton of plasma weapons and terminator armor they seem to have.
The Rock is home to several
Fallen Angels FOUL BETRAYERS FROM THE NINE TRAITOR LEGIONS WHO ARE NOT AFFILIATED WITH THE DARK ANGELS IN ANY WAY in the process of being redeemed (by which we mean they are tortured and made to repent for their treason before being finished off) and formerly Luther who recently escaped after an invasion by daemonic forces of Chaos. The Lion El'Jonson himself is also hidden away somewhere in the depths being cared for by the Watchers in the Dark, although the Dark Angels are unaware of their Primarch's continued existence; except for Luther, who has long since gone insane but constantly rambled about the Lion's impending return.