Answer
The doubters are probably misunderstanding the meaning of a "scientific theory" vs. a hypothesis. In everyday life, these two terms are often used interchangeably, i.e., a "theory" may be nothing more than a speculation. However, in science, a theory is "a synthesis of a large body of information that encompasses well-tested and verified hypotheses" ("Summary of Terms", page 17 of textbook) and is on very solid ground.
Nonetheless, though scientists do not doubt the broad features of the theory, they are prepared to accept refinements to that theory as new evidence presents itself.
For example, Einstein’s general theory of relativity works better than Newton’s gravity equation to explain things such as perturbations in Mercury's orbit, and the bending of light as it passes by a massive body.
