Light is composed of electromagnetic waves, which therefore need a medium to manifest themselves, so its speed can be isotropic only relative to the medium and not also relative to celestial objects, including the Earth, which move in the medium. Physicists have devised various experiments to demonstrate that the speed of light is not isotropic relative to the Earth, but without success. To justify this negative result, some physicists have hypothesized that material objects, as a function of their speed relative to the medium, undergo a time dilation and a length contraction, making the speed of light appear isotropic even if it is not, so that said isotropy would be only apparent. Instead, I have actually demonstrated, through the CMBR, that the speed of light is not isotropic relative to the Earth and, therefore, that this isotropy is only apparent. To explain what the medium is and how it works, I developed a theory that I called Space Quanta in Expansion, which then identified the medium with expanding space, and which explains how photons and material objects move in the medium. Among other things, it argues that the expansion of the Universe is not caused by two forces, one that makes material objects move away, which would be the so-called dark energy, and another that makes them move closer, which would be gravity, but is caused by a single force, the one that makes the space quanta expand and which could explain dark energy. Therefore gravity is an apparent force.