Scientists just confirmed the existence of "Dark Energy", a mysterious repulsive force that acts in opposite to gravity. As the distance increases, the attractive gravitational force decreases but this mysterious repulsive force increases. This repulsive force is pushing galaxies apart; the greater the distance the greater the repulsion. Scientists today do not know what this "Dark Energy" is, but they know that it is causing the entire universe to expand at an increasing rate.

For the first 7 bln years after the big bang the expansion of the universe slowed down because the attractive gravitational forces were stronger than this repulsive force. However as the distances between the galaxies increased the attractive gravitational forces weakened while this repulsive force became dominant. This made the expansion of the universe to enter an accelerating phase:    

Do you remember how a rose opens up? That is, the outer petals move outwards more than the inner petals? Imagine that those petals have galaxies on them and that we are at the center of the rose. Now imagine this rose opening up; the farther out the petals are the faster their recession away from the center (where we are). Well this is exactly how the universe expands around us; the farther out galaxies are from us the faster their recession away from us.

The future of the universe has three possible scenarios depending on this dark energy. The first scenario is if this dark energy is constant over time; then the expansion of the universe would continue accelerating forever. After a hundred billion years or so from now most of today's observable galaxies will not be visible (they will disappear from our sight but they will continue to exist) This is called the Big Chill:      

The second scenario is if this dark energy increases with time then the universe will experience a catastrophic runaway expansion. Within 100 billion years or so from now every galaxy, star and atom in the universe will be ripped apart!!! This is called the "Big Rip":      

The third scenario is if this dark energy decreases with time. This eventually leads to a slowing of the expansion of the universe followed by a recollapse. This is called the "Big Crunch". In some ways this scenario resembles the Big Bang in reveres:       

Mysterious matter called Dark Matter provides six times more gravity than Normal Matter (six times more than Stars, Planets...); however Dark Energy dominates the Universe:    

Dark Matter

Big Rip or Big Crunch? Future depends on this mysterious Dark Energy:

Dark Energy

NASA recently ruled out  the second scenario (No Big Rip; see also: Universe Today). This leaves the universe with only two possible endings: Big Chill (constant expansion) or Big Crunch, depending on what this Dark Energy turns out to be.