Dark Energy

What happens when you throw a ball high into the air? Earth's gravity slows it down, and eventually it falls to the ground. Astronomers thought the universe would work the same way. The universe began 13.8 billion years ago with the Big Bang. So they expected gravity, with everything pulling on everything else, to slowly slow down the universe's growth. By the way, the Big Bang was not an explosion that sent fragments flying out from one point. It was the moment when space itself began to swell everywhere at once.

But in 1998, scientists observing distant supernovae saw something they could hardly believe. Instead of slowing down, the universe was expanding faster and faster. It was as if the ball you threw had switched on a rocket engine and kept accelerating upward.

That meant some huge force exists that beats the giant brake called gravity and pushes the whole universe outward. Scientists had no idea what was causing it. So they gave it a name that means "dark and unknown": dark energy.

The names are so alike that it's easy to mix up dark matter and dark energy. Both are invisible, but they do opposite jobs. Dark matter has weight, and it pulls things tightly together. You can think of it as the "glue of the universe."

Dark energy, on the other hand, pushes space itself outward and moves galaxies apart. It acts as the universe's repulsive force (a pushing force). If you want an analogy, dark matter is the universe's brake pedal and dark energy is its gas pedal.

On very large scales, that gas pedal is now beating the brake. In an expanding universe, the farther away a galaxy is, the faster it moves away (Hubble's law). For galaxies far enough away, that speed can even exceed the speed of light. But this doesn't mean the galaxies are flying through space at ultra-high speed. It means the space between them is stretching. Inside galaxy clusters held tightly together by gravity, in the solar system, and in our own bodies, nothing expands.

More precisely, the leading candidate for dark energy is not some special lump of stuff. It is energy built into space itself. Physics calls it the "cosmological constant" or "vacuum energy." That said, DESI observations in 2024-2025 showed signs that dark energy may be weakening over time, so the debate over what it really is continues. Ordinary matter gets thinner as the universe grows. Dark energy has an unusual property: as space stretches, its total amount keeps growing by exactly the amount of new empty space.

This is why dark energy has come to dominate the universe over time. In the early universe, matter was packed densely and gravity was strong. As the universe grew, the density of matter dropped, but dark energy kept an almost constant density.

To uncover the secrets of dark energy, scientists are mapping the universe with the latest space telescopes. The universe's final fate depends on it: dark energy might eventually tear everything apart (the Big Rip), or let the universe cool and freeze (the Big Freeze).