Are we talking about the big crunch? Which will perhaps happen?
My theory is that our universe is held together by the gravitational mass of the black holes. In all galaxy centers, there is a black hole which keeps everything from expanding due to the pull of the universe. One day, the universe will reach its maximum, and begin to implode, thus all of the planets who originally felt the gravitational pull of the universe will now feel more attracted to the massive black holes in the center of the galaxies. This will mean that any matter will be destroyed, and soon only black holes will be left. Eventually I believe that they will all form one supermassive black hole, which, under the weight of the whole universe will implode upon itself, thus creating a space so infinitesimal that even the slightest stir will cause it to explode. BOOM. It explodes, new big bang, nature recreates itself, perhaps as a parallel universe or what not. Thoughts?