Some extracts from Chris Impey, University Distinguished Professor and Deputy Head of the Department of Astronomy at the University of Arizona, in his book How It Began: A Time-Traveler’s Guide to the Universe (New York and London: W. W. Norton and Company, 2012):
When astronomers first became aware that the universe is expanding, they realized there was tension between light and dark. If the expansion had been much more rapid than we observe, the gas would have thinned out too rapidly for gravity to form stars and galaxies. On the other hand, if the expansion had been much slower than we observe, the gas would have congealed into a single massive object, or the universe might have collapsed too quickly back to its original state for stars and galaxies to form. (208)
But, relatively recently and to the extreme astonishment of scientists, it’s been discovered that the expansion of the universe is actually speeding up.
What causes the acceleration? That’s the most profound mystery in cosmology. The evocative term given for the agent accelerating the universe is dark energy. But it’s more of a placeholder for ignorance than a physical description. Dark energy acts in opposition to gravity, causing space-time to expand at an ever-increasing rate. Other than that, almost nothing is known about it. Physicists are intrigued and chagrined by dark energy because it represents new physics and is not part of the “standard model” of fundamental particles. (210-211)
The universe has a driver, and it’s a bad and anxious driver; maybe it’s never driven a universe before. Its feet are on the brake and the accelerator at the same time, but the braking foot is weaker and gets tired. Initially, the universe is barreling along with speed decreasing due to strong pressure on the brake. As the brake eases, the accelerator, which has always been pressed down, begins dominating. The universe accelerates because the brake is applied with diminishing force. Dark matter is the brake; dark energy is the accelerator. For the first 8 or 9 billion years of cosmic history, dark matter decelerated the expansion. About 5 billion years ago, dark energy eclipsed the diminishing tug of dark matter, and we’ve been accelerating ever since. Two entities that we don’t understand control the universe, and the normal matter that includes us is just along for the ride. (211)
Maybe this little video will put you in the appropriate mood: