The short answer is ‘Dark Energy’, but then the obvious next question is “what’s dark energy”? It’s named that basically because we know very little about it. We can observe that the Universe is expanding by measuring the speeds of distant galaxies; we find that all are receding from us, and that the further away they are the faster they are receding. In addition, by looking at the nearby universe Astronomers have managed to determine that the universe recently (a few billion years ago) entered an era of accelerating expansion. This acceleration must require an energy source, and we call it dark energy.
One idea is that dark energy might be associated with the vacuum energy of free space. Due to quantum mechanical fluctuations (a whole other topic) even completely empty space has a non-zero energy associated with it. This being the driving force would make sense because as space expands you get more empty space so the dark energy driving expansion would increase, leading to accelerating expansion. However, attempts to connect the two have thus far been unsuccessful. This may well be because currently there is no theory of quantum gravity, and Cosmology is fundamentally about gravity, and the vacuum energy (probably source of dark energy) is fundamentally a quantum effect.
This page was last updated June 27, 2015.