From Andrew Zimmerman Jones,
Your Guide to Physics.
FREE Newsletter. Sign Up Now!

Definition: Dark energy is a hypothetical form of energy that permeates space and exerts a negative pressure, which would have gravitational effects to account for the differences between the theoretical and observational results of gravitational effects on visible matter. Dark energy is not observed, but rather inferred, along with dark matter, as a probable explanation for these effects.

The term "dark energy" was coined by the theoretical cosmologist Michael S. Turner.