Gravity is way too weak a force to hold something as massive as a galaxy together.

Well it obviously does. If not gravity, then what ?

No one knows. But you are never going to find the answer if you keep inserting explanations that make no sense.

The current model, based on gravity alone holding things together,
balanced against the effective centripetal and centrifugal forces as
in any orbit, works just fine.

Lots of things go faster than 186,000 miles per second.

OK, so name just one.

Matter from exploding stars; some rogue planets--especially ones knocked free from close to their stars.

None of these things go anywhere near light speed,
let alone exceed it.

But when something goes that fast it exits the universe--that is why it cannot be observed.

Flash ! - the universe is everything by definition.