dark energy

Since the 1920s, observations of distant galaxies reveal that our universe was expanding. Astrono- mers thought that gravity, which draws matter together, would eventually slow, stop, or even reverse this expansion. But in 1998, astronomers determined that the expansion of the universe is actually speeding up! There is a mysterious pressure working in opposition to gravity, pushing matter apart. Scientists call it "dark energy."

Definition: a mysterious pressure that is causing the expansion of the universe to accelerate
Discovered: in 1998 by two independent teams of astronomers
Measured: In 2002, the Wilkinson Microwave Anisotropy Probe determined that more than 70% of the universe is made of dark energy.
Cool fact: The force of dark energy seems to be growing stronger as the universe expands!

Image credits: main image, © AMNH.