"After the Big Bang, the universe expanded and cooled down. And we expect this expansion to gradually slow down because the universe has things like galaxies inside it, and they are attracted to each other by gravity. But in the past 25 years or so, observations have shown precisely the opposite: The expansion of the universe is speeding up. It’s accelerating. This is the concept of , and it points to something that we are missing in our description of the universe. Dark energy is sometimes seen as this mysterious, magical source of energy that accelerates the expansion of the universe. But this isn’t really the core of the problem. We can cook something up for dark energy — just as we do for dark matter — and hope we’ll detect it later. It’s not particularly satisfying, but we do this, we’ve done it before. Really, ."