I imagine one type of probability space as an extremely wide and extremely high tower of Jenga blocks under bombardment from an arbitrarily large number of bored children wearing jetpacks. The vertical, y axis represents the passage of time. The higher you go up that axis, the further you travel into the future. The x and z axes represent probability. The further away from the origin a block is, the more improbable.
Since not everything is possible, and thus, probability is finite, the tower has an edge all around it. Blocks are constantly falling off that edge, taking with them all the blocks that are above them. That means the more improbable events are removed from probability space as time goes forward, and eventually, there will be no more probable events remaining, and thus , with the end of all probable events, the universe will end.
But the falling blocks do not do so in an orderly way. Now and again, islands form in the tower, events that have lower probability than the “mainland” of probability, but since they are linked together with many other events, do not fall as easily as they “should”.
That is the answer I see as to how a complex universe such as our own could possibly form out of a simple, random, and disorderly past such as that of the big bang. It is complex, surely, but it will not last. Eventually the last block at the summit of the island in probability space will fall, taking us with it. And the second law of thermodynamics is upheld.
Image by Sergio Flores Rosales