What causes gravity? where does its energy come from? will it ever lose energy?

Gravity is the effect that Mass has on the fabric of space. It's not truly energy, it's a force (there's a subtle difference).

So far as we know, it should *always* have the same amount of force.

Actually those are very good questions.

All we know is that gravity is created by any mass or energy concentration, as described in General Relativity. Prior to it, it was believed that mass alone would create gravity, but it turns out that the concentration of any form of energy (not just mass, as expressed in a quantity called "tensor of energy and momentum") has an effect on space-time geometry (a tensor called "metric tensor") and thus immitates a gravitational field (something going "straight forward" takes a curved trajectory when compared with the absense of any interactions).

As to whether it is constant in time, that is a very good question. http://en.wikipedia.org/wiki/Theory_of_everything It is supposed that at extremely high energies (such as those right at the beginning of the Big Bang), such energies make all interactions (electromagnetism, nuclear forces, gravity) look more or less the same, but this also has implications in the intensity of interactions. But these are very speculative, since there is no experimental evidence (laboratory results) that would suggest that this would be true; it's just a consequence of pushing the mathematics from what we already know to see what we'd get supposing what it would be possible from them.

Again, let me stress this very important point: it's just speculation, and there is no physical evidence that the intensity of gravity changes with time.

The source of gravity is stress-energy. This includes the mass of matter.

This is a well-understood property of General Relativity, and has met every practical test that has so far been invented.

As far as we know, energy is conserved. In order for energy to be conserved, the gravitational field must have negative energy.

There is no evidence for a change in the relationship between energy and gravity in the past 13 billion years.

There are two phenomena happening in the universe right now. First, The "Anti-Matter" (the one that expands the universe and serves as the pushing energy), and the Second phenomenon, the "Gravity" (the one pulling objects together).

The source is still unknown. These two phenomena were only defined and calculated.

Gravity is a property of the mass of an object, and for some reason, gravity causes objects to be attracted to one another. Scientists aren't completely sure why it happens, only that it does.

No one knows

All we know is anything with mass has gravity, the earth has a large mass so will always have some sort of gravitational force

Gravity is caused by mass and it one of the four fundamental forces in the universe. No one knows what "causes" the fundamental forces, but our universe is defined by their existence.

As soon as we figure out what "space" is made of we will know what "gravity" is made of. They are very closely related.