That’s true. Nor has anyone shown it to be right. In science, hypotheses have to be supported by evidence. The process is observation, testable hypothesis, empiric test.

Mathematicians, aka theoretical physicists, write gravity-only equations. When their equations do not describe what they measure, they add terms to their equations to balance them.

For instance, in their equations, the added term for dark matter attracts and slows the outer reaches of galaxies—slowing them so they stay with the galactic center. Those outer reaches do not equally and oppositely attract the added dark matter and in their minds it stays where they had put it.