The paper presents an alternative explanation for the existence of nearly massless scalar fields in the universe, which are suggested by evidence of the accelerated expansion of the universe and the time-dependent fine-structure constant. The authors propose that the mass of these scalar fields depends on the local matter density, leading to couplings to matter of order unity while satisfying tests of the Equivalence Principle (EP). In regions of high density, such as Earth, the mass of the fields is large, suppressing EP violations. In regions of low density, like the solar system, the fields are essentially free, with a Compton wavelength much larger than the solar system's size. This model predicts significant EP violations in space, which could be detected by upcoming experiments like the SEE Project, STEP, Galileo Galilei, and MICROSCOPE. The paper also discusses the thin-shell mechanism, which ensures that the chameleon-mediated force between large objects like planets is suppressed, satisfying solar system tests of gravity. The authors conclude that their model provides a viable explanation for the observed phenomena and suggests that future experiments will provide strong evidence for its correctness.The paper presents an alternative explanation for the existence of nearly massless scalar fields in the universe, which are suggested by evidence of the accelerated expansion of the universe and the time-dependent fine-structure constant. The authors propose that the mass of these scalar fields depends on the local matter density, leading to couplings to matter of order unity while satisfying tests of the Equivalence Principle (EP). In regions of high density, such as Earth, the mass of the fields is large, suppressing EP violations. In regions of low density, like the solar system, the fields are essentially free, with a Compton wavelength much larger than the solar system's size. This model predicts significant EP violations in space, which could be detected by upcoming experiments like the SEE Project, STEP, Galileo Galilei, and MICROSCOPE. The paper also discusses the thin-shell mechanism, which ensures that the chameleon-mediated force between large objects like planets is suppressed, satisfying solar system tests of gravity. The authors conclude that their model provides a viable explanation for the observed phenomena and suggests that future experiments will provide strong evidence for its correctness.