open, closed & flat universe
open universe: Any model of the Universe which does not contain enough matter to halt its expansion.
closed universe: a universe in which there is sufficient matter to halt the expansion that began with the big bang and to cause the universe to collapse again; the visible matter is only 10 percent of the matter required for closure but there may be large amounts of dark matter
flat universe: A universe where the geometry of space does not curve but is flat like a tabletop.
