Question

What is the fundamental source of all energy in the universe ?

Answer 1

The energy from the sun is what sustains the solar system. But this is only a small part of the universe. So it will be inaccurate to say that the source of all energy in the universe is the sun. 
The universe is made up of several other galaxies including our own milky way. But looking at these components of the universe, they are made up of stars. Considering that all other stars are similar to our sun, which by the way is also a star, then we can put it that their collective energies is what constitutes the energy source for the universe. 
But here we have only considered what we know about the universe, since man has been able to explore only a minute fraction of the universe, it wont be fair to argue that this is absolute. There may be other energy sources out there that have not yet been discovered, but for now, the idea behind the summation of celestial energies should suffice.