Early Galaxies of the Universe
One of the major hypotheses on which modern cosmology is based is that the Universe originated in an explosion called the Big Bang, in which all energy (and matter) that exists today was created. Since then, assume cosmologists, the Universe has been expanding outward. This expansion was discovered by Edwin Hubble in his study of Doppler-shifted light; just as the pitch of a train whistle appears to drop as the train moves away from you, so does light from stars appear more red if the stars are moving away from Earth. Hubble discovered the astonishing fact that all stars are moving away from us; Earth not being the center of the Universe, this instead tells us that everything is moving away from everything else, as it has been for the fifteen billion years of the Universe. This expansion is often modeled as dots (stars) drawn on a balloon being blown up. The Big Bang theory replaced the "steady-state" model of the Universe, which held that the Universe was largely unchanging (and was not expanding or contracting).

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 10–43 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.