Big Bang Explained...  a Cosmological Theory
  of the Origin of the Universe... the Big Bang is a unifying theme of
  cosmology and is based  upon EXPANSION, a sort of an burst of
  space itself that happened everywhere, much like the surface of a
  balloon expanding - it happens everywhere at once. The Big Bang
  is a hypothesis 1st proposed by Belgian cosmologist Georges
  Lemaitre in 1931, with undeniable evidence of the theory with the
  discovery of cosmic microwave background radiation in 1964 by
  Robert Wilson and Arno Penzias of Bell Labs .
Misconceptions:
  1.  was not really an "explosion" from a particular location into a preexisting void.
  2.  model doesn't describe the Big Bang itself, only what happened afterward.
  3.  Universe was not size of a orange, i.e, energy & heat were just more tightly packed.
  4.  not everything expands. Gravity overpowers expansion and galaxies remain same size.
   
An analogy:
  a.  You live on the surface of an inflating 2D balloon; with inflation the distance between points is
           increasing, thus the surface of the balloon is expanding; distance to remote galaxies is
           increasing, but the galaxies themselves are not expanding, only the space between galaxies
           is expanding.
  b.  In reverse: any given region of the Universe shrinks and all of the galaxies within it get closer,
           until they smash together at start of Big Bang age of our Universe.
 
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Age of the Universe...
     Scientists can guess the universe’s age by using its expansion rate, known as the Hubble constant. The expansion itself is caused by dark energy, and the great proportion of dark energy to matter has resulted in an accelerating expansion rate. Combined with the density of matter, researchers can determine how fast the universe expanded and when that expansion began.

     To determine the density, composition, and expansion rate of the universe, various missions have measured the residual thermal radiation from the Big Bang, known as cosmic background radiation. These missions performed calculations using those data, and have collaboratively arrived at an age of 13.798 billion years, give or take about 37 million years. Physicists have measured the distortions caused by two orbiting galaxies, and found that the universe is both flat and infinite.

Size of the Universe...
     A best guess for the size of the observable universe is about 46 to 47 light-years away in every direction—assuming, by the cosmological principle, that the universe behaves under the same forces throughout. That observable universe forms a sphere centered around us, so we are essentially located at the center of our universe.

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