How can the rate at which the universe is expanding accelerate?

If the universe or the latest incarnation of the universe we inhabit started with a Big Bang, I can understand why matter is hurling away from the point of singularity and the universe is said to be expanding.  

If that motion result from the Big Bang, intuitively, I would expect that the energy of that explosion would dissipate over time and that the rate that the universe expands would decline. 

If I am wrong, and the absence of impediments and friction in the vacuum of space negate the dissipation of energy, then I would expect the universe to expand at a constant rate. 

Observations from the Hubble Telescope tell us that the rate at which the universe is expanding is accelerating.

How can this be? For this to happen, a new release of energy is required. Where does it come from?