What energy source in the universe provides the inexhaustible power for such forces as gravity and magnetism?

Power and related energy are not inexhaustible when talking about useful energy/power. Useful energy will eventually (billions of years from now) totally run out in our universe.

The universe has something called positive entropy, which simply means that as time passes and on an average useful energy is being converted to useless energy (which, even though it is not properly defined as such, is what entropy turns out to be). In the end, many astrophysicists agree, the universe will burn out like a candle and its absolute temperature everywhere will be 0 degrees Kelvin. In which case there will no longer be useful energy.

Gravity and electro-magnetism are two of the four fundamental forces in our universe. Strong and weak atomic forces, which keep atoms together and govern decay rates and types, are the other two.

Of the four, gravity is the weakest force. You can tell gravity is the weakest because if it were stronger than the other three, a person jumping off a ten story building would go through the cement sidewalk instead of splatting all over it. The electro-magnetic forces of the cement atoms are what splatter the hapless jumper all over.

As to the source...all these forces were created out of the big bang, which occured 13 billions years ago or so. As the universe cooled over the first few seconds or so, the amorphous mass-energy of the early universe began to sort of congel into the quarks and forces we know today.

These are two separate things. Lets start with magnetism or electro-magnetism. Think of a spring and a tight rubber band. Now imagine two balls that are held together by the tight rubber bands and at the same time being spread apart by the spring. Thats kind of like how the forces in electro-magnetism work with atoms. Now think of a sheet of rubber stretched across a large opening and a heavy object like a bowling ball placed in the center of it. Now imagine a smaller object such as a marble being shot across the rubber sheet - it would not go straight across but would curve in toward the bowling ball as it approached that area. If it went in close enough it would go in a circle around where the bowling ball was located. Thats kind of how gravity works on large objects like stars and planets. These forces do not come from some super energy source but rather from the way the universe is actually constructed - it comes from the universe's geometry.

Gravity and magnetism are not giving energy, they are just static forces, fields. Just like a spring, or a rubber band: those can apply a force, but until motion occurs, no energy is being released. You gve energy to the system (be it a spring, a rubber band, a pendulum) by compressing, stretching, of hoisting the system.

But surely, there is *potential* energy, in the fact that objects are attracted to one another, but are kept from falling in but orbital velocity, and one can ask where that initial potential energy comes from. And the answer to this is, essentially, the big bang. That huge explosion that created the universe also pushed objects away from one another, giving them the initial potential energy.

solar?