Maya

Well according to some it's actually 6015 years old (4004 + 2011) since there was "light"
http://en.wikipedia.org/wiki/Ussher_chronology

However the Creationists have it that a day was not a day and (shh) the Cosmologists agree (kinda) Hence "Cosmological Time" (1) as we now think that time is non linear...

But to put it in a very VERY small nutshell: The Big Bang Theory
Observations discovered that the galaxies are uniformly receding away from us (Hubble 2)
and thus at one point in time they all would have been coincident. All the mass/ energy of the Universe as a single MASSIVE "monoblock" or singularity.

Which went bang.

The speed of the receding galaxies and the distance travelled provided the first approximation to the age of the universe. Since reinforced using Microwave telescopes on the ground and in space to measure background temperature of space and thus the age of the Universe measured by the rate of cooling from its very hot beginnings.
13.75 ± 0.11 billion years
1/ http://en.wikipedia.org/wiki/Age_of_the_universe
2/ http://en.wikipedia.org/wiki/Hubble%27s_law
3/ http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation#Relationship_to_the_Big_Bang

But it's much, MUCH more complicated than that as there may have been existance of 'something' before the Big Bang and there are theories that the Big Bang is little more than a quantum event in a much bigger Universe.
http://en.wikipedia.org/wiki/Ekpyrotic_universe

But now we are in the realms of string and M theory and way beyond my capabilities.
(A joke here about bears of little brane.)

I would add that there are good reasons to believe that our Universe is just one in an infinite series of MultiVerses.

Finally is the theory that our Universe is just too perfect and therefore *must* be a very complex computer simulation constucted by hyper intelligent alien identities.
http://en.wikipedia.org/wiki/Simulation_hypothesis

Reality is Illusion "Maya"