When you sum up all the red shifts of a large piles of galaxies they are receding from us at speeds faster than can easily be explained using the Hubble model.

When you sum up all the red shifts of a large piles of galaxies they are receding from us at speeds faster than can easily be explained using the Hubble model. One theory for this is a repulsive energy of a form we have not yet observed, dubbed "dark energy", which is purported to be causing accelerating expansion of the universe since the Big Bang. More here: https://en.wikipedia.org/wiki/Dark_energy https://en.wikipedia.org/wiki/Dark_energy