Whether the expansion will continue forever is a topic of debate. If the Universe has more than a certain amount of mass (termed the "critical mass" for obvious reasons) gravity will eventually slow the expansion to a stop and initiate a recollapse of the Universe (termed the "Big Crunch"). On the other hand, if the mass is insufficient, then the Universe will continue to expand indefinitely. Interestingly our best measurements of the total mass of the Universe are inconclusive when it comes to deciding between a "closed" and an "open" universe.
The quantity of dark matter in the Universe will partly determine this. The observed mass of the Universe is not nearly enough to account for several calculations of the motions of large bodies of stars. This "missing" mass is called dark matter because it cannot be seen, and it is believed to comprise around ninety percent of the Universe's total mass. Though there is little debate about the existence of dark matter, physicists are unsure as to the form of dark matter. Candidates include neutrinos (particles whose mass value is not well known), space rocks and comets, and "small" black holes created in the first seconds of the Universe. The proponent of the small black hole theory is Stephen Hawking, a theoretical physicist at Cambridge who has in recent decades been one of the leaders in topics of astrophysics.
Because the Universe is expanding, it must, as we look backward in time, have been smaller and smaller to the initial singularity, a point at which general relativistic spacetime curvature is infinite. As in the vicinity of a black hole singularity, the laws of physics in the early Universe (the first 1043 seconds, what is known as Planck time) were quite strange because gravity, which determines the curvature of spacetime, had not yet separated from the other four fundamental forces.
It is interesting to note that Stephen Hawking and Roger Penrose proved that the Big Bang was actually the beginning of time. This in itself seems contradictory, for how can anyone talk about the beginning of the thing that determines beginnings itself? They did it, however, by defining time as that in which events are determined by past events. Events may have occurred prior to the Big Bang, but because those events had no effect on events after the Big Bang, they cannot be considered part of this time. Therefore the Big Bang was the beginning of time.