There are a few scientific theories that have attempted to describe the universe using the laws of physics. In all these theories the event of Big Bang is taken to be the starting point (Scroll down and listen to the sound of the Big Bang.) How did they come to this conclusion? Here is how they did it:
In the first two decades of 20th century astronomers observed that all other galaxies are constantly moving away from our Milky-Way galaxy, and studying the light coming from these galaxies they realized that everything is moving away from everything else. The way scientists can determine this using light is made possible by the same laws that make an ambulance siren sound different when it is moving away or toward us. Light too changes in its frequency when it is coming from a moving source; this is called the red-shift in physics. In 1927 Georges Lemaitre, a Belgian physicist and Roman Catholic Priest, postulated that this motion of the universe can be traced back to a point called the singularity, hence the beginning of the story of Big Bang by a Catholic priest. In 1920s Edwin Hubble provided more experimental evidence of expansion and red-shift from his astronomical observations; he also found the mathematical law that gives the speed of this expansion.
It was later shown that this explosion and the subsequent acceleration can be derived from Albert Einstein’s theory of General Relativity. It turned out that Black-Holes and Big Bang are logical consequences of Einstein’s field equations.
In later decades scientists were able to collect more accurate data using more advances telescopes and experimental devices. They found out that the universe is expanding with an acceleration. The best experimental data, which is frequently tested and verified, comes from measurements that captures the residues from the Big Bang explosion. This data which is known as CMBR, Cosmic Microwave Background Radiation, is a record of the thermal radiations caused by Big Bang and has filled the whole of space. The following image is a visual of this background radiation.
You want to hear the sound of Big Bang? The information in the above image is converted into an audible file and sounds like this:
(Source of Audio: http://faculty.washington.edu/jcramer/BBSound_2013.html)
The most recent theories of physics such as String Theory and Loop Quantum Gravity are attempts at unifying the laws of nature, especially Quantum Mechanics and Gravity, but they all agree on the event of Big Bang.
However, there are physicists whose research has taken them to more surprising conclusions. A group of physicists at the Perimeter Institute for Theoretical Physics have speculated that perhaps the Big Bang has never really happened! The idea is this: Though we appear to be in a universe that appears to have come from a big explosion, it is possible that the explosion has never really happened; instead our universe is projected out as a hologram. The idea of a holographic universe is also present in string theories but need more than 10 dimensions for their existence. But the new idea of these physicists at PI does not need higher dimensions. In their theory, our 3 dimensional universe is really the surface of a 4 dimensional sphere enclosing a 4 dimensional singularity at its center. The singularity never explodes; instead it projects outwardly its own infinite possibilities, and one of them is our universe.
A summary of their work is available here:
A very important advice would be to be aware of the ontological status of these various models of the observable universe. These models, though accurate and obtained by hard work, are nonetheless models. People get excited about new science stuff, such as quantum or big bang, and try to make paradigm shifts or base their ideologies and self-perceptions on these models. But this is wrong. Scientific theories describe a world that is not identical with the world of human experience, remembering that science is a small part of human experience. Science itself is one among the many human traditions; it is a human activity, and thus its results are derived and known by us and are not meant to govern or restrict us. If we are to base our spiritual or cosmic understanding on science, then we would have to change our ideas every decade or so. These models will change as better approximations and measurements become available. We see it in health science very often: One day peanuts are good and then they are not; egg is good and then it is bad, etc.
The universe of science is a mathematical model filled with endless approximations to interpret controlled experiments. The best example I can give you about the status of these models is this: What science produces is like a set of slides captured by a camera. When these slides are played they give the illusion of real experience but when we look at them they are just dead slides. Remember, film slides are a representation of reality; but reality is not the film slides.