where exactly the gravity come from ? what holds on the whole universe ?

We are not certain is the honest answer. Mass curves space (possibly via the Higgs field) and curved space is gravity.
The predictive theory and maths does not fit the inverse square law which we observe,
The Universe is actually flying apart slowed by gravity.

There's a couple of working theories behind gravity.
There are experiments looking for the "Graviton" - a hypothetical particle that's a quantum-gravity particle... (I think it's poppycock, but there are smarter people than me who are pursuing it...)
The working theory is that mass bends space. This bending is what we *feel* as gravity. The closer to the source of gravity, the more space is bent. (It makes intuitive sense...)
**Why** mass bends space... is unknown. The Higgs Boson that was indirectly detected using the LHC a couple of years ago are beleived to be the reason *why* matter has the property of mass, and the Higgs Field is thought it's the means with which it affects space.... but, research continues...

No one knows.

When YOU figure it out, the Nobel Prize in Physics will be your reward.

"Where exactly the gravity come from?"

No one knows exactly. What agrees with all measurements / tests, is that spacetime is shaped by mass, and curves it. And new theories may arise, that make better predictions.

"what holds on the whole universe?"

Nothing. Gravity is not a force, and we know that the expansion of the Universe is accelerating.

It's stagnant energy that is responsible for mass, which in turn is responsible for gravity.

If you'll recall one of the big discoveries that Einstein made was that mass and energy were really the same things. Mass was just a different phase of energy, much like ice is a different phase of water. Mass is really a form of energy that is stuck in a place. Energy normally travels at the speed of light, unless it is in the form of mass, then by definition it always travels at less than the speed of light. So mass is really a stagnant form of energy, unable to travel at the speed of light because it's stuck to a section of space. When energy gets stuck like this, it is called mass. And when this mass is stuck in a section of space, it creates a stress and tension in the space, and that part of space becomes depressed, which creates curvature and warpage inside that space.

So basically after a mass warps and curves space, objects travelling through that space follow a curved trajectory. It doesn't matter if the object that's travelling through has mass itself or is massless, they all follow the curved path.

the mass of an object pulls things towards it. we have gravity, but we are too small for it to do much and its cancelled out by earth's gravity. Nothing holds on the whole universe as it is constantly drifting through the cosmos in a single direction-driven by the "Big Bang"

I can answer your first question. I can surely tell you that the more mass the more gravity. Lets look at how stars and planets are born. Pieces of dust and matter clump up and the more matter together creates a higher amount of mass making more gravity present. Eventually all the matter will be so big like earth that the center heats up to extreme temperatures.

Physicists believe that the Higgs Boson is the reason for gravity because it gives mass to particles around it. It was also found in 2013 at CERN.