I've never gotten into a TV show before, but 'Lost' has found me.
Last night's long-awaited episode? Oh my! I've never felt that look on my face before; one of thrilled, trembling incredulity. And the same expression was frozen on my 2 roommate's faces.
Here's a dope-ass speculation I found on what that dark cloud was all about:
"The "monster" is compressed dark matter, the "living" -- in so far as its sentient and has form -- product of a scientific experiment. It communicates through the computer and is embodied in the black cloud. Dark matter is supposed to effect gravity, so it would be able to suck people into the air just by coming into contact -- or near contact -- with them, as was the case with the pilot. It supposedly changes the rotational pattern of galaxies, so it would explain the unlikely isolation of the island, the fact that compasses don't work (or do, and the sun is rising and setting in the wrong place). Through the entering of the numerical sequence into the computer, the dark matter is kept compressed. If the numbers don't get entered, the dark matter expands into a black hole, and goodbye galaxy."