The universe is expending.

This is what I know from Discovery channel.
At this time, all the galacies are moving apart at high speed.
And the speed isn't constant,
It actually speeding up (moving faster and faster)
The Discovery Channel does not say why it speeding up.

This is what I thought.
Consider the momentum of a galaxy MV, (mass X velociy)
M=total mass of a galaxy, V=speed of that galaxy.
If M is reducing (mass loss in all Star in the fusion reaction,
where mass are converted into energy)
That is why V have to increase to keep the momentum constant.

But there is a giant black hole in the centre of each galaxy.
Soon or later all the Star will fall into that black hole.
It is at the centre of the black hole where energy are
converted back to mass.
Soon or later the reverse process will take place.
That is the Mass of the galaxy will increase and the speed of
the galaxy will start to slow down.
And the universe will start to compress toward a single point.
When all the galacies are compressed into that single point,
perhaps another Big Bang will happen again.