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What is dark energy?

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Quick answer

Dark energy is a mysterious force thought to make up most of the universe and to be driving its expansion to accelerate. We can't see or directly detect it, but its effect — pushing galaxies apart ever faster — is measurable across the cosmos.

Key things to understand

  1. It appears to make up roughly 68% of the universe.
  2. It acts like a repulsive force, accelerating cosmic expansion.
  3. It was discovered in 1998 from distant supernova observations.
  4. Its true nature is one of the biggest mysteries in physics.
  5. It's different from dark matter, which pulls things together.

People also ask

What is dark energy?
An unknown form of energy filling space that appears to push the universe apart, causing its expansion to speed up over time.
How do we know dark energy exists?
Observations of distant exploding stars showed the universe's expansion is accelerating, which requires some repulsive energy to explain.
What's the difference between dark energy and dark matter?
Dark matter's gravity pulls things together; dark energy does the opposite, pushing space apart and accelerating expansion.

Part of the guide: Space & Astronomy →