Wikipedia comes to the rescue again.
http://en.wikipedia.org/wiki/Ultimate_fate_of_the_universe
http://en.wikipedia.org/wiki/Ultimate_fate_of_the_universe
http://en.wikipedia.org/wiki/Large-scale_structure_of_the_cosmosRepulsive force
Starting in 1998, observations of supernovae in distant galaxies have been interpreted as consistent with a universe whose expansion is accelerating. Subsequent cosmological theorizing has been designed so as to allow for this possible acceleration, nearly always by involving dark energy, which in its simplest form is just a positive cosmological constant. In general dark energy is a catch-all term for any hypothesised field with negative pressure, usually with a density that changes as the universe expands.