Why is the expansion of the universe speeding up?

For most of the 20th century, scientists assumed the universe's expansion was slowing down. Every galaxy pulls on every other galaxy with gravity, and gravity should act like a brake, so the expansion that started with the Big Bang should gradually decelerate. In the 1990s, two rival teams set out to measure that braking. They tracked a special kind of exploding star, the Type Ia supernova. It always peaks at almost the same brightness, so how dim it looks tells you how far away it is.

Both teams got the opposite of what they expected. The distant supernovae were fainter, meaning farther away, than a slowing, decelerating universe could explain. Announced in 1998, the result meant the universe's expansion isn't slowing down: it's speeding up. Three scientists behind the discovery, Saul Perlmutter, Brian Schmidt, and Adam Riess, later shared the 2011 Nobel Prize in Physics for it.

Something has to be pushing space apart to cause that acceleration, and scientists call it dark energy. Today's best estimate says dark energy makes up 68 percent of everything in the universe, more than ordinary matter and dark matter combined. Yet it is extremely diffuse. It dominates only because it fills all of space evenly. What dark energy actually is remains unknown; the leading candidate is something called the cosmological constant.

That idea isn't new. Einstein invented a cosmological constant back in 1917, not to explain acceleration, but to force his equations to describe a static, unchanging universe, since without it his math predicted expansion or collapse. He called it an 'ugly thing' and dropped it once Hubble showed the universe really was expanding. Only in 1998, decades later, did scientists bring the same mathematical term back, this time to explain why that expansion keeps accelerating.