An important question today in cosmology is how much mass the universe contains. If there were no matter filling the Universe, the Universe would expand forever and the recession velocity of objects at rest with respect to the expansion of the Universe would not change as the Universe expands.
We know, of course, that the Universe is not empty but filled with matter, and ordinary matter through gravity attracts other matter, causing the expansion of the Universe to slow down. If the density of the Universe exceeds a certain threshold known as the critical density, this gravitational attraction is strong enough to stop and later reverse the expansion of the Universe, causing it eventually to recollapse in what is known as the “Big Crunch.” On the other hand, if the average density of the Universe falls short of the critical density, the Universe expands forever, and after a certain point the expansion proceeds much as if the Universe were empty. A critical Universe lies precariously balanced between these two possibilities.