According to Stephen W. Hawking, Black holes are rips in the fabric of space of the universe. They can be neither be visible to our eyes nor are they detectable by even the most powerful telescope -- the Hubble Telescope -- so far created by man. According to Hawking also, the Black hole are nothing but dead stars. The collapse of the star leads to singularity.
The black hole may be described as the final stage in the death of a star before the point of singularity is reached. For Roger Penrose, a British mathematician, the collapsed star would form a singularity where time ceased and the laws of physics no longer applied. In other words, it would defy the laws of gravity to the extent of having mass but no dimension. Likewise, space-time and light wouldn't just be drawn into a hole; they would be wound up infinitely tightly to the point where they disappeared.
Hawking began studying Penrose's ideas and essentially developed his own and like many such great ideas, this was essentially simple. Hawking asked himself what would happen if a black hole could somehow reverse itself. And what if the expanding universe was no more than a huge collapsing star in reverse? This was where his theory of the "big bang" began (Strathern 52-3). The Big Bang does not consist of a huge primeval atom sitting in empty space and then exploding outwards. Most people still have this image, in which the galaxies are like fragments of an exploding bomb, hurtling outwards through space.
Instead the Universe itself is encompassed in a sort of cosmic egg - a black hole, but on so huge that the bending of space-time within it is too small to be measured by any astronomical instrument on Earth. By expansion, it has become a very low density black hole, in which light now follows very gently curving orbits at the horizon (White 80). How did the universe started? In order to predict how the universe should have started off, one needs laws that hold at the beginning of time such as Einstein's theory of relativity and Penrose's singularity theorem but the singularity theorem indicates that the gravitational field becomes so strong that quantum gravitational effects became important, thus using the quantum theory of gravity to discuss the early stage of the universe (Hawking 133).
In the classical theory of gravity, which is based on real space-time, there are only two possible ways the universe can behave: either it has existed for an infinite time, or else it had a beginning at a singularity at some finite time in the past. In the quantum theory of gravity, on the other hand, is a third possibility whereby the time direction is on the same footing as directions in space, it is possible for space-time to be finite in extent and yet to have no singularities that formed a boundary or edge. Space-time would be like the surface of the earth, only with two more dimensions. The surface of the earth is finite in extent but it doesn't have a boundary or edge (Hawking 135). Now that Hawking has an idea about how the universe started, he predicted that the universe is continuously expanding and it is extremely probable that the present rate of expansion of the universe is almost the same in each direction.
In an expanding universe in which the density of matter varied slightly from place to place, gravity would have caused the denser regions to slow down their expansion and start contracting. This would lead to the formation of galaxies, stars, and eventually even insignificant creatures like ourselves. Thus all the complicated structures that we see in the universe might be explained by the no boundary condition for the universe (Hawking 140-1). In conclusion, Hawking expresses that as long as the universe had a beginning, we can suppose it had a creator. But if the universe is really completely self-contained, it would then have neither beginning nor end.
"My goal is simple. It is a complete understanding of the universe, why it is as it is and why it exists at all". - Stephen W. Hawking Hawking, Stephen. A Brief History of Time: From the Big Bang to Black Holes. New York: Bantam Books, 1988.133,135,140-1. Strathern, Paul.
Hawking and Black Holes: The Big Idea. New York: Anchor Book, 1997.50-3. White, Michael and John Grib bin. Stephen Hawking: A Life in Science. Great Britain: Penguin Books, Ltd., 1992.80..