Why does gravity make the universe recollapse?
Gravity makes the universe recollapse in much the same way that gravity brings an object thrown upwards back to Earth. Galaxies are flying away from each other due to the expansion of the universe, but they are also all gravitationally attracted to each other. The more mass, the more the overall gravitational attraction between them. If this attraction eventually overcomes the outward expansion, then everything will recollapse. If the attraction just exactly balances the expansion, the expansion will eventually become infinitely slow. If there is just not enough mass in the universe and the attraction never overcomes the expansion (analogous to an upwardly-thrown object exceeding Earth escape velocity), then the expansion will keep going.
