Galaxies are accelerating away from each other 5% to 9% faster than had previously been thought, say scientists.
The discovery made by the Hubble Space Telescope is a puzzle because it conflicts with measurements of the Cosmic Microwave Background (CMB), the ancient afterglow of the Big Bang.
Dr Adam Riess, from the Space Telescope Science Institute in the US, said: "This surprising finding may be an important clue to understanding those mysterious parts of the universe that make up 95% of everything and don't emit light, such as dark energy, dark matter and dark radiation."
One possible explanation could be a new type of subatomic particle that may have changed the balance of energy in the early universe, scientists believe.
Astronomers used Hubble to measure the distances to stars in 19 galaxies with more precision than ever before.
It was Hubble observations that made the bombshell discovery in 1998 that the universe was not only expanding, but expanding at a faster and faster rate.
An invisible force called "dark energy" is hypothesised to be repelling the galaxies but its nature is still a complete mystery.
The findings will appear in a forthcoming issue of The Astrophysical Journal.