Big
Bang
The Big Bang theory is the
prevailing cosmological model that explains
the early development of the Universe. According to the
Big Bang theory, the Universe was once in an extremely hot and dense state
which expanded rapidly. This
rapid expansion caused the Universe to cool and resulted in its present
continuously expanding state. According to the most recent measurements and
observations, the Big Bang occurred approximately 13.75 billion years ago, which is thus
considered the age of the
Universe. 
After its
initial expansion from a singularity, the Universe
cooled sufficiently to allow energy to be converted into various subatomic
particles,
including protons,neutrons, and electrons. While protons
and neutrons combined to form the
first atomic nuclei only a few minutes after the Big Bang, it would take
thousands of years for electrons to combine
with them and create electrically neutral atoms. The first
element produced was hydrogen, along with
traces of helium and lithium. Giant clouds of these primordial elements would
coalesce through gravity to form stars and galaxies, and theheavier elements would be
synthesized either within stars or during
supernovae.
Source: wikipedia, NASA
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