Why is the universe expanding?
Space itself stretches, a momentum left by the Big Bang, now pushed faster by dark energy.
Space stretches, galaxies don't fly through it
The common picture of an explosion hurling debris outward is wrong. Galaxies are not racing away through space; the space between them is itself getting longer, like dots on a balloon drifting apart as it inflates.
Because every region stretches at once, there is no center. From any galaxy, all the others appear to recede, and the farther one sits, the faster it pulls away.
The evidence: redshift
In the 1920s Edwin Hubble found that light from distant galaxies arrives stretched toward the red end of the spectrum. The stretch grows with distance, exactly what you would expect if the space the light crossed had expanded along the way.
Why it keeps going, and speeds up
Measurements of distant supernovae in 1998 revealed the twist: expansion is not coasting to a halt but quickening. The leading explanation is dark energy, an energy woven into space itself that makes up roughly 70 percent of the cosmos.
What the universe is made of
The stuff we can see and touch is a thin rind on the whole. Whether expansion races on forever or one day reverses depends on dark energy, the part we understand least.