They come from N.O.W.H.E.R.E.  They are the Zero Agents, the Cyphermen, they work for Division Zero doing the multiverse's dirty work.   They work behind reality, never talking, never revealing their secrets.  It is said that they are all clones, genetically programed to be emotionless, logical, organic machines.  Totally efficient, cold and calculating. The Zero Agents travel in the cracks of reality, culling the weak planes of existence from the strong.  They stop the diease of contaimination from jumping from one dimension to the next.  Its not pretty, but it needs to be done.  They take no joy in their grisly work, they require no thanks. 
   The Men In Green employ impossibly advanced training and technology to give even the very gods themself a shudder of  fear at their arrival. 
   They survey the Cosmic Orrery and prevent clashes and catastrophies.    From there well protected base, N.O.W.H.E.R.E   they use anamathamatical equations to divine when planes crash, and whether this will be harmonious or disasterous.   If they deem it a disaster, then Division Zero sends in the MIGs.  Annilation is green.