Dark Energy

Dark energy is a mysterious force that seems to be pushing the universe apart. Scientists do not know what it is. They only know it is out there, because the universe is behaving in a way that nothing else can explain. Dark energy makes up about 68 percent of everything in the universe, which is more than all the stars, planets, and galaxies combined.

To understand dark energy, it helps to know one thing about the universe. It is growing. Ever since the Big Bang, about 13.8 billion years ago, galaxies have been moving away from each other. For a long time, scientists thought this growth was slowing down. Gravity pulls things together, so it should also be pulling the universe inward.

Then in 1998, two teams of astronomers measured faraway exploding stars called supernovas. They were trying to find out how fast the growth of the universe was slowing. They got a shock. The universe was not slowing down at all. It was speeding up. Galaxies are flying apart faster and faster every year.

Something had to be pushing them. Scientists gave this something a name: dark energy. The word "dark" does not mean it is black. It means we cannot see it and do not understand it.

Dark energy is not the same as dark matter, even though the names sound alike. Dark matter is invisible stuff that pulls things together with gravity. Dark energy does the opposite. It pushes things apart. About 27 percent of the universe is dark matter. Only about 5 percent is the regular matter that makes up you, your dog, planets, and stars.

So what is dark energy? Nobody knows. Some scientists think it is a kind of energy built into empty space itself. Even a box with nothing in it may hold a tiny bit of this pushing force. Multiply that by the huge empty space between galaxies, and the push adds up. Other scientists think our understanding of gravity might be wrong on very large scales. A few think dark energy might even change over time. New telescopes, like the James Webb Space Telescope, are hunting for better answers.

The future of the universe depends on what dark energy does next. If it keeps pushing, galaxies will drift so far apart that the night sky of the distant future will look almost empty. Dark energy is the biggest unsolved puzzle in science today.