dark energy

Since the 1920s, observations of distant galaxies reveal that our universe was expanding. Astronomers 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."

Image credits: main image, © AMNH.