Some had hoped it might be just an illusion. But it looks like dark energy is real and here to stay, as astronomers "image" the mysterious entity in action. In 1998, astronomers found that distant supernovae were dimmer, and thus farther away, than expected. This suggested that the expansion of the universe is accelerating – and "dark energy" was named as the culprit. Since then, astronomers have struggled to explain what dark energy actually is – leading some to speculate that it may not exist at all...