A schematic diagram showing the birth of the universe amid the big bang, the subsequent expansion of the universe, and an eventual re-contraction ending in a "big crunch." The white circles represent matter in our universe subject to the "arrow of time," according to which a wineglass, after it falls off a table, will never reassemble itself and jump back up on the table. The black circles represent matter associated with a hypothetical inverse arrow of time.
Viewed from the perspective of this matter, the concepts of an expanding and contracting universe would be reversed from our customary experience. Isolated bits of this reverse-arrow matter (the black circle near the left end of the figure) might be present in the observable part of our universe in the form of "dark matter."
