gravity is not something that is exactly the same everywhere. It is something that is moves around the orbit of larger gravities (e.g. the earth has a center of gravity that rotates around the sun, and each of them has a different strength and nature)
The fact that this is maintained is no more impressive than looking at a computer for 10 years and realizing that the computer doesn't break into a million pieces for no reason. That is, things don't change by themselves. They need a catalyst to change.
As soon as we find a proper catalyst to change the gravity, we could make it less stable, just like if we found a proper catalyst to change the state of a computer (e.g. a hammer), we could make it less stable (e.g. be smashed by a hammer)
But the same goes for things that were created naturally.
Now, as for your whole thing about everything must come from something else... Well, no duh.
A decent majority of people who believe in the big bang theory and the like acknowledge that, even though they have a theory on how the universe was created, they don't know how that super-condensed piece of matter came to be, and why it exploded. That isn't the important part of the big bang theory, though.
The most important part of the big bang and evolution theories is everything that happened after the bang was initialized, that is, as soon as the matter started to explode, that is the important part.
It shows a theory on how the rocks and gasses that make up the galaxy may have come to be where they are, and how they might have changed to become habitable.