Dark Energy

Dark Energy is a hypothetical type of energy believed to be the cause of Universal expansion.  It is believed to fill up about 70% of the Universe.  It exerts a negative pressure on its surroundings which is behind the Universe's expansion.  To explain the acceleration, "vacuum energy", an energy in empty space, was proposed.  As the Universe expands, more vacuum and dark energy are formed in the new space that is created.  This, in turn, adds to the negative pressure that the Universe is exerting.  The Dark Energy's density is nearly unchanged as the Universe expands but the density of Dark Matter decreases.  This steadily increases the domination of this energy. This chain reaction, unless stopped by a different factor will lead the Universe to a Big Freeze or a Heat Death, possibly preceding a Big Rip.