That the universe is expanding. It makes perfect sense too, but just in case let me give proof. The astronauts placed mirrors on the moon when they first went up. Scientists have been shooting lasers up to the moon every so often, probably every year. They measure the length of the laser beam when it reflects off the mirrors. Each year the moon gets a little further away. I can't remember how much it is something like a centimeter or 2. Therefore, proving the universe is expanding.
The universe is and probably always will be expanding until it breaks off into 2 universes or collides with another universe. These are obviously not proven to be what actually happens, but I would like to believe in the theories that are currently in place supporting this as how the universe works.
Studies show that the earth is slowly moving away from the Sun. There is other data indicating that the rest of the planets in this solar system are moving away from the sun. This indicates that this solar system is expanding in its entirety. Theorists believe that this phenomenon is also happening throughout our universe. The collapse of the universe may happen just as quickly as it began according to big bang theorists. According to modern theory and measurements though, we are still in the expansion phase.
Scientists have noted a "red shift" in measuring the speed of nearby galaxies. That means light coming from these galaxies is shifted more towards the red spectrum. Therefore, those heavenly bodies are moving further away from each other and further away from a central point. The universe is expanding if galaxies move away from one point with all due haste.
All evidence right now and all acceptable theories in physics, as we stand at the present time, say that the universe is expanding and will one day die a heat death. Of course this is being challenged by several other hypothesis, but none have yet to gain status of a theory.