# Why do we need the concept of dark energy to explain the expansion of the universe?

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?

• Note that observations seem to point to not merely on-going expansion, but accelerating expansion (i.e. it's expanding faster now than in the cosmological past). Try explaining that in a model without dark energy. – dmckee --- ex-moderator kitten Mar 3 '12 at 1:56
• hmm... interesting. If that were the case (accelerating expansion), it would make sense to rely on dark energy. How would the astronomers computer the expansion rate from past? – Shreedhar Kotekar Mar 3 '12 at 4:54
• – user4552 Apr 26 '18 at 21:15

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 !