I, like most of my brethren here, prefer no explanation. It lends itself to all sorts of possibilities, lets the viewer decide for themselves, makes it SCARIER IMO.

One other thing I don't think anyone else touched on is that if the world fell apart as shown in the movies, no one would probably ever know why it happened EITHER. You'd get a bunch of wild theories flying around in the 1st few days & then when everything fell apart....no answer. To me, it's the most realistic & really puts us in the same mindset as the characters themselves.