What observation has prompted astronomers to ponder the possible existence of dark energy

Name: What observation has prompted astronomers to ponder the possible existence of dark energy
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Observations of supernovae at large redshifts have shown that they appear fainter than expected, indicating that the expansion of the universe is accelerating. This suggests the existence of an unknown energy density, or "dark energy," which is thought to be responsible for this acceleration.

What Observation Has Prompted Astronomers to Ponder the Possible Existence