The relativity/dark matter combination, however, is just the most successful theory of gravity; it's not the only one.

Isaac Newton had run into the same problem with his own theory of gravity some 250 years earlier.

They were unable to find any variation from Einstein's theory of gravity.

There will be another better theory of gravity in the future, for example.

The first was a dynamic theory of gravity, which I have worked out in all details and hope to give to the world very soon.

My first guess would be centrifugal force but this too uses the theory of gravity.

"Because we do not have a theory of strong gravity," the head of the theoretical division said.

There have been many different attempts at constructing an ideal theory of gravity.

Could there be more mass in the universe than we are aware of, or is the theory of gravity itself wrong?

There are some metric theories of gravity that do not fit into the above categories, but they have similar problems.