The Universe is expanding, but that might be a temporary thing. Some observers say that the rate of expansion is increasing. Some say that the mutual gravitational attraction of all the matter and energy in the universe is slowing the expansion of the Universe down, At some point in the future the Universe may or may not stop expanding and start contracting, ending in a Big Crunch – the Universe would in this case be closed. This is illustrated on the diagram below by the curve

The parameteris the ratiowhereis the critical density, the density the universe would have to keep expanding, but constantly decelerating, with the expansion coming to a stop at infinite time, andis the present density of the universe. The density of the universe, all things considered, seems to be suspiciously close to the critical density, which is aboutKg per cubic m but recent evidence suggests the rate of expansion may be increasing, so the Universe is open. Open Universes are illustrated by the three top curves on the diagram above.

To complicate things the Universe is pervaded by a mysterious substance called Dark Matter, about which we know very little, but which may contribute greatly to the mass of the Universe.