Why 13.7 billion years

I do understand how the universe spawned from nothing, since the total energy is 0 (assuming that we live in a flat universe) and all that positive/negative energy from gravity. But why 13.7? There has to be a better explanation than that. Why can't it be longer than that exact moment - exact point at 13.7 billion years ago?

Good question! Bear with me here, because we have to talk about some complex stuff. After all, probing the nature of the universe isn't easy..

The number can be calculated a number of ways, using what is called 'Concordance Cosmology'. This means that there are 3 (known) ways to test for the age of the universe.

I believe the first was using knowledge of fusion combined with the temperature of the cosmic microwave background. In essence, you arrive at a temperature for photons after the Big Bang. Then you use knowledge of the initial 'scale factor' (think size) of the universe as compared to the scale factor today.

As photons travel this immense size, in an expanding universe, they will cool because they lose energy. The predicted temperature of photons (see Wien's approximation) based on this theoretical calculation was exceedingly close to the temperature of the cosmic microwave background [CMB] as measured most recently by WMAP.

The value of time required to lower the temperature of the CMB may also be calculated, again using the scale factor of the universe. When you do this calculation, time = ~13.7 billion years.

It turns out this was a sort of lucky result, as recent evidence ~1998 showed that the expansion of the universe underwent a cosmic "jerk" (a change in acceleration), meaning that the universe was a decelerating expansion for the first ~6.5By, and then a dynamic transition caused it to begin accelerating as it expands. This is a result from examining distant supernova, which it turns out leads to another method of calculating the age of the universe.