We are not sure of what is at the origin of our universe, and we probably nevre won't be it, but we can formulate assumptions. Most probable is that of the big-bang that here :
To the XXE century, physicists and astronomers made a success of the exploit to apprehend the whole of cosmos : they realized that it was neither immutable, nor eternal. The Universe has a history : it appeared there is approximately 15 billion years during an episode which lasted nearly three hundred and thousand years.
Let us start with a short outline. All would have started with a large flash, a titanic explosion of light, matter and energy. That would have revealed the elementary particles (like the quarks or the neutrinos). While interacting in complex and fabulous reactions, these particles would have to form the protons, the neutrons and the electrons. The protons and the neutrons would have been assembled, forming the cores of helium and hydrogen atoms. The electrons would have joined these cores to form helium and hydrogen atoms. Lastly, by nuclear reactions such as those which are held in the Sun, the chemical elements were formed all starting from hydrogen and helium.

But which is this large flash of which all left ? It is located in a very short moment, the first moment of the universe, which lasts only 10-43 second, and which one called the era of Planck. During this moment, all the physical sizes like time or space are confused and the universe is only one single atom of energy, indivisible, ultra-dense and ultra-heat. At this time, the forces unify. First particles are created under the effect of a primitive force, incredibly powerful.
The expansion of the universe starts. It's not a movement within a fixed framework, but it is a general movement, a kind of swelling of the universe, as if a balloon were inflated. In only one blow, the « diameter » of the universe is multiplied by 1050 and its volume is thus multiplied by 10150. At this point in time the primitive force is divided into three forces which with gravity (present with the primitive force) form the four fundamental forces : gravity, electric interaction, strong interaction and weak interaction. In same time, the primitive particles disintegrate to form the fundamental components of the matter : quarks, electrons, neutrinos.
Then one arrives at moment 10-6 second. The quarks are assembled and formed the atoms. At this point in time the matter becomes transparent, which will allow 300 000 years later in the light of escaping and dispersing. Then, this extremely fast movement slows down considerably and one will need billion years so that the atoms are assembled to form planets and stars, then the life.

This scenario seems justified by three observations : on the one hand the expansion of the universe is a reality : it was discovered about 1920 by Edwin Hubble which noted that the galaxies moved away. In addition, the studies show that helium and hydrogen prevail in the universe to 99 %. Lastly, one discovered (by chance) in 1964 the radio operator radiation. It acts of a kind of background noise of the universe, which would be the vestige of the original explosion.
This said, which occurred before moment 10-43 second is still unknown for us. It is precisely what currently prevents us from defining the big-bang as the reality of the facts.

But especially, a great question arises, in connection with the expansion of the universe. There are two possible theories :
- Is the universe is infinite. In this case, the expansion will last eternally. Consequently, energy in each point of the universe will decrease (One of the fundamental laws of physics explains that the total energy of the universe is finished, determined, constant. Thus if the universe grows, energy is more and more divided very into remaining in the same quantity, with the result that it is weaker in each point. It is as a plastic balloon which one inflates : the more it is inflated, the more the color clears up oneself). The universe will thus end up being a gigantic world whose energy in each point is almost null. It will be thus a died and cold universe : the temperature will go down while approaching more and more, but without never reaching it, the absolute zero.
- Is (that's my favourite theory) the universe is finished. In other words, the expansion cannot last indefinitely. In this case, the universe will end up reaching a critical size. The consequence will be that after having grown, the phenomenon will be reversed : the universe will narrow and redensifier until becoming again what he was before moment 10-43 second. The universe will finish like he started. Thus, there could be a new big-bang. And all the process will start again. This theory leads us to think that before the universe, there was : universe. Our universe would be only one version of an infinity of universes, which eternally follow one another in a cycle « big-bang, expansion, anti-expansion, anti-big-bang ».

More informations :
Les trois premières minutes de l'univers (The first three minutes of the universe), Steven Weinberg, Le Seuil, 1978.
Dernières nouvelles du cosmos (Last news of the cosmos), Hubert Reeves, Le Seuil, 2002.