It was formerly believed that if all material things disappeared out of the universe, time and space would be left. According to relativity theory, however, time and space disappear together with the things. It is a very useful theory, with amazing consequences, but can only be applied to frames of reference which move with uniform motion i.e. constant velocity. The general theory expands on the special theory, making it applicable to all frames of reference, without regard to their motion. It also recasts gravity as a field rather than a force.
Gravity is the means by which an object with mass warps space and time.
It's this warping of space that feels like a force to an observer. Anything moving slower than this can not be accelerated up to that speed, but it could reach a speed very close to it. But, if an object could reach the speed of light, it's length would shrink to nothing. As length becomes shorter for a moving object, its mass increases and time slows down. Relativity forced a reexamination of the physical properties of energy and momentum.