What Is Dark Energy, and Why Is It Important?

If you're wondering what dark energy is, that's understandable; scientists are still wondering that very same thing.

It's probably the most mysterious idea in all of physics, and a good deal less understood than dark matter—a substance which is itself hidden in the shadows. Dark matter can at least be mapped and understood as physically distributed in the universe, much like ordinary matter. But dark energy has only recently been able to be investigated at all.

So, what is dark energy? Well, it's been a few things over its lifetime.

How We Got Here

After Einstein, physicists were fairly certain about how the universe was evolving: the Big Bang threw all the matter outward, as an explosion does, after which gravity naturally attracted that matter to itself and slowed the outward expansion.

This was, itself, a big admission. Einstein initially proposed a static universe, but ...