Why can't the expansion of the universe be thought as the Big Bang itself still in progress? Why do we need to introduce dark energy? The Big Bang was powerful, and that explosion itself could still be continuing right?
The observed expansion of the universe was the initial inspiration for the idea of the Big Bang, and for about 70 years, the Big Bang alone was enough to explain the observed expansion. As dmckee points out, the acceleration of expansion is what inspired the idea of dark energy.
The initial observations that indicated expansion of the universe were observations of supernova at reasonably large cosmological distances. Type Ia supernovae, which are produced from the explosion of a degenerate white dwarf, are "standardizable" candles. (They are often incorrectly called standard candles, but Type Ia supernovae can have different luminosities, but from nearby supernovae it is known that a supernovae that have a longer period of time during which they are close to peak brightness are also intrinsically brighter at maximum.) Using Type Ia supernovae to measure distances, it was found that very distant supernovae were slightly farther away from us than we would expect from their redshifts, and that this was systematically true. The simplest explanation was an unknown repulsive energy in the universe, called dark energy.
This is not the only proposed idea. Some people have argued that gravity might operate differently over long distances, and that this could produce the observations that have been interpreted as accelerating expansion. Dark energy is the most widely accepted explanation though, as it also explains a variety of other observations, such as the flatness of the universe. Now, astrophysicists just need to figure out what it is.
We need to introduce the concept of dark energy because we know the universe started to accelerate in a "recent epoch" of the cosmic evolution. The common matter (baryonic matter) and the dark matter are gravitationally attractive. If the universe is composed only by matter (normal matter plus dark matter) the expansion of the universe must be decelerating. So there must exist a component that do not behave like matter: it must be gravitationally repulsive. We don't know what that strange component is, but we call it dark energy. The most impressive is that almost $70 \%$ of the universe is dark energy !