What Is Dark Energy? The Force Accelerating the Universe

Dark energy is the name physicists give to the mysterious something that is making the universe’s expansion speed up. It appears to make up about 68% of everything in the cosmos — and yet we cannot see it, touch it, or directly detect it. It is one of the deepest mysteries in science.

The discovery that shocked physics

For most of the 20th century, cosmologists assumed the universe’s expansion must be slowing down, held back by gravity. In 1998, two teams measuring distant supernovae found the opposite: the expansion is accelerating. Something is pushing the universe apart faster and faster. That discovery earned the 2011 Nobel Prize — and it demanded a new ingredient in the cosmos.

What dark energy does

• It acts like a kind of repulsive pressure filling all of space.
• Unlike matter or dark matter, it doesn’t clump — it seems evenly spread everywhere.
• As the universe grows and matter thins out, dark energy’s influence grows, so the acceleration increases over time.

What could it be?

Honestly, nobody knows. The leading ideas:

1. The cosmological constant — energy inherent to empty space itself (vacuum energy). This fits the data well, but the value it should have according to quantum theory is wildly larger than observed — a mismatch called “the worst prediction in physics.”
2. Quintessence — a dynamic energy field that changes over time, rather than a fixed constant.
3. Modified gravity — the possibility that our theory of gravity itself is incomplete on cosmic scales.

The cosmic budget

Put together, the universe is roughly:

• 68% dark energy
• 27% dark matter
• 5% ordinary matter (everything we can see)

In other words, the atoms that make up stars, planets and people are a rounding error in the cosmic budget. Missions like ESA’s Euclid telescope are now mapping the universe to pin down what dark energy actually is.

Related: what is antimatter?

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