How Do We Know It’s There?
Space telescopes studying the Big Bang’s relic radiation (the cosmic microwave background) have found the universe is almost perfectly flat. That means light travels in straight lines unless it encounters mass. Scientists say this only really makes sense in a universe brimming with some kind of dark energy.
Dark energy would also explain why our universe’s expansion is accelerating. Although we’ve known for about a century that galaxies are receding from ours, scientists long speculated that matter’s gravitational heft would eventually slow cosmic expansion — maybe even reverse it, culminating in what’s called a Big Crunch. In 1998, however, astonished astronomers discovered the opposite. A certain kind of exploding star, called a supernova, turned out to be fainter than expected in the distant past, indicating that the universe is ballooning at an ever-faster rate, and has been for nearly half of its 13.8 billion-year existence. At the current clip of cosmic growth, the distance between galaxies will double in 10 billion years.