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Why cosmic expansion is speeding up

What’s the mysterious “stuff” that causes this acceleration?
RELATED TOPICS: DARK ENERGY | SPACE PHYSICS
Coma-Galaxy-Cluster
Fifteen year ago, two teams of astrophysicists announced that distant objects were farther away than theoretical models said they should be, and so the universe must be expanding at a faster rate. Before that research, scientists thought all the matter in the universe would eventually stop cosmic expansion and lead to a contracting universe — after all, the force of gravity attracts objects to one another.

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