By observation we know that the Universe continues his expansion. The evidence collected so far, confirm the permanent expansion of the Universe theory. But at this point, not all astronomers are agree, as many argued that the dark matter (hypothetical matter of unknown composition, which does not emit or reflect enough electromagnetic radiation to be observed directly using current technology, but its existence inferred from the gravitational effects that they cause in the stars or galaxies) could exert enough gravity force to curb this expansion and ensure that all this matter is going to compress again. This possible involution of the universe is called the "Big-Crunch" or the Great Implosion.
Since the universe is expanding, galaxies are moving away from each other. Astronomers deducted that in the past had to be a moment in which they were very close to each other. Thus began taking shape the "Big Bang" theory, which is currently the most widely accepted to explain the concerneance to the birth of the universe. Describes the expansion of space-time from an initial point where it was concentrated all matter and energy in the universe. After the Great Explosion, the primordial matter was composed of protons and neutrons loaded with great energy. These elementary particles joined to form atoms. The Universe thus experienced a rapid period of cosmic inflation that swept through all initial irregularities becoming bigger and becoming more stable, ie colder and less dense. It is believed that nowadays the Universe continues cooling down.
After the Big Bang variations in the distribution of mass occurred. As a results of this, galaxies were formed. If the universe had been completely uniform, galaxies have not been formed.