The history of dark energy appears to begin with the Big Bang — the instant of creation for all matter and energy, space and time.
The Big Bang took place about 13.7 billion years ago. In the first hundred-billionth of a trillionth of a trillionth of a second, the newborn universe began a period of hyperactive growth that's known as inflation. In a tiny fraction of a second, it grew 1050 times larger — a "1" followed by 50 zeroes. It "inflated" from smaller than an atom to bigger than a golf ball. That doesn't sound very impressive, but it was enough to "flatten" the geometry of the universe. In essence, it was like blowing up a balloon so large that any portion of its surface looks like a flat tabletop, not a curve. This geometry, in fact, is important evidence of the existence of dark energy.
Scientists aren't sure what caused inflation, but it may be related to dark energy — and perhaps it was the same dark energy that we see in the modern universe.
Yet after this initial tiny burst, inflation disappeared and the universe resumed a more leisurely expansion rate.
After about 400,000 years, the universe had cooled enough that particles could combine to form atoms. In this environment, the universe became transparent, like a cloud that has spread out enough to let you see through it. This era also left an "imprint" that still permeates the universe today — a background radiation that glows at a temperature of just 2.7 °C (4.9 °F) above absolute zero.
Yet this so-called cosmic microwave background, which is one of the confirmations of Big Bang theory, is not completely smooth. Instruments aboard balloons and satellites have detected tiny variations in temperature, which are produced by variations in the way matter was distributed. In other words, the early universe was slightly "lumpy," with the denser lumps of matter forming the seeds from which the first stars and galaxies took shape.
Over the next billion years or so, the universe continued to expand and cool, with enormous amounts of hydrogen and helium lumping together to give birth to galaxies and their individual stars.
Observations indicate that this early universe was dominated by dark matter, a mysterious form of matter that produces no detectable energy, but that reveals its presence through its gravitational pull on the visible stars and galaxies. In fact, dark matter "outweighs" all the normal matter and energy in the universe by about six to one.
And so the universe continued on. The universe continued to expand from the impetus of the Big Bang, but the gravity of the dark matter and visible matter was trying to pull everything back together, slowing the rate of expansion.
But about five billion years ago — nine billion years after the Big Bang — the universe began to expand faster. Dark energy began to dominate the universe, causing space itself to expand faster, carrying the galaxies along with it. Since then, the expansion rate has continued to speed up, as though someone were pushing a little harder on a cosmic accelerator pedal.
If this trend continues, over the eons the galaxies will grow so far apart, and their motion away from each other so fast, that each galaxy will become like a little "island universe." Anyone living inside a galaxy won't be able to see all the other galaxies, so the universe will become a landscape of perpetual darkness — each galaxy isolated from all the others in a runaway universe.