Gravity, at the most macro level, is the force of attraction between different objects based on their relative masses. One idea about the effect of the force of gravity is a subset of the big bang theory known as the oscillating universe theory. According to this idea, while the universe expanded from a single point, the force of gravity will eventually overcome the force propelling the expansion of the universe: in the contraction that follows, the universe will return to the pre-big bang state.
Because physicists have documented the movement of different celestial bodies in relation to one another, they have established a number of equations and rules that predict how different bodies will move in the presence of other bodies. However, there are a number of anomalies in these observations: bodies that follow trajectories that physicists would normally predict requires the presence of much more matter than exists around these anomalous bodies. This has led physicists to theorize about "dark matter," a material that exerts the gravitational force that equations predict is necessary for the body's behavior but in a form that scientists cannot yet detect.
When Albert Einstein made his contributions to existing ideas about relativity, his findings had implications for gravitational forces. According to his ideas, the space-time continuum is a generally curved surface in which massive bodies that exert great gravitational forces, such as the sun, create dips and valleys. Scientists have confirmed these findings in a number of observations, including the fact that light will bend around bodies of great mass, such as the sun.
Another aspect of Einstein's theory of relativity is setting the absolute boundary on the speed at which energy or matter could move at the speed of light in a vacuum. One theoretical consequence of this was the existence of the gravitational bodies known as black holes. In these areas of space, matter is condensed into such a density that its gravitational force creates an event horizon: a point at which not even matter or energy traveling at the speed of light can escape being sucked into the hyper-dense body.