So some scientists believe the universe must have been slightly lumpy to begin with. In 1992, the satellite Cosmic Background Explorer detected faint ripples in the microwave background, which may indicate this was true. As astronomers gaze into distant space, they see galaxies hurtling away from us in all directions at astonishing speeds. The further galaxies are away, the faster they seem to be moving. Most astronomers are sure this means that the universe is expanding.
Some scientists think so.
This is called the open universe idea. Others argue that gravity is already beginning to put a brake on the expansion. If there is, indeed, a lot of dark matter out there, its combined gravity may eventually slow the expansion to a standstill. Once all the momentum has gone, gravity may finally begin to pull the galaxies back together again. If and when this happens, the gravity of the galaxies will draw them closer at ever faster speeds and the universe will shrink almost as if the Big Bang were happening in reverse.