Why is there acceleration in universe expansion?

Firstly, The universe is estimated to have started accelerating about 5 billion years ago, this is because matter was much closer together back then, and so gravitational forces were greater.

There is this stuff called Dark energy (http://en.wikipedia.org/wiki/Dark_energy), this is theorised to be what it causing the expansion of the universe to accelerate. But nobody knows what it is at the moment :)

why is there acceleration in universe expansion?
Same force from Big Bang still exists - universe less dense now - make sense? I can elaborate if need be.

Just like the theory of general relativity, and the theory of quantum mechanics, both of which we could not be where we are today without them