But we "know" about them.

We don't know about them, they are hypothetical and merely assumed in many theories. They create problems with theories when introduced as particles, which is why string theory makes them strings. But nobody knows if strings exist, and nobody knows if the extra spatial dimensions that string theory predicts exist, and nobody knows if the extra particles that string theory predicts exist. Gravitons are not fundamental to the theory of loop quantum gravity, and only appear at low energy approximations.

"Since the cosmic microwave background is a form of light, it exhibits all the properties of light, including polarization." - Primordial wavefunction overdensities conflate forming the barriers of the universe. The residual overdensities outside the "marble"-sized unit of universe-energy could just as easily form gravitational waves and be massive on the scale of our universe without requiring energy escaping the marble* to travel faster than light. That makes several assumptions of course, but this way the energy of our universe may be paid back to that same primordial (non-trivial) wavefunction to the end of heat in our universe, explaining why expansion speeds up as heat is dispersed further and quicker toward the end of time.

So the universe didn't begin when 2 clouds of primordial density collided, but rather when they tried to separate.

*the marble meme is based on the idea that all the energy in the universe was once compressed into a ball about the size of a marble (prior to 'expansion' ldo)