This observation led to the theory that some mysterious, gravitationally repulsive dark energy must be behind the rising expansion of the cosmos - although we don't really have the foggiest idea what that is.

BBC: Dark discussion ahead for Europe and US

And dark energy may turn out to be almost the mirror opposite of gravity - a repulsive rather than an attractive force, and one that strengthens with distance rather than weakening.

BBC: Getting the measure of dark energy