THERE’S A MYSTERY AT THE heart of physics.

Two decades ago, in 1998, cosmologists discovered that the universe is not just expanding—a discovery of the early 20th century—but that the rate at which it’s expanding is getting faster.

That’s not what they expected to find, but it made a kind of sense. If the expansion of the universe is accelerating, there needs to be a cause; not knowing exactly what that was, physicists called it “dark energy.”

In theory, dark energy interacts through gravity, is spread out homogeneously through the universe, and is not particularly dense. If you total up all of the forces that make up the universe, it would account for 68.3 percent of matter and energy.

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