In science, a theory is "the explanation that best fits the observed facts." A scientist is ALWAYS ready to ditch any theory in favor of a better one. It's possible to have two equally valid theories; for instance, regarding gravity. One theory is that gravity involves an exchange of particles, like the other three basic forces of nature. Another (Einstein's) is that gravity is caused by the bending of the space-time continuum. Either one explains the visible manifestations of gravity.