Cosmology is currently facing some major challenges. In addition to dark matter and dark energy, the James Webb Space Telescope has brought the issue of "impossible" galaxies to the fore. In the present work, we put to the test the following conjecture: something simple eludes us, and the various problems mentioned are interrelated. Our proposition is that, on the cosmological scale, it is appropriate to take a value of the speed of light c lower than its standard value c0 in vacuum. For this, we will write c = cc (subscript c for "cosmological"). This defines an optical index nc = c0 /cc. We account for this "refringence" by a Shapiro effect extended to the scale of the universe, described by its average density ρu and its equivalent gravitational radius Ru. Remarkably, universes with indices greater than two are entirely conceivable, and their characteristics are close to those we determine for our own. The velocities v of celestial objects are estimated from redshifts in ratios of the type v/c, where the speed c of light is usually taken to be equal to c0. With an equal v/c ratio (all things considered, only the v/c ratio has any meaning), dividing c0 by a certain factor α lowers the velocities v without postulating the existence of dark matter nor dark energy. Taking into account the problems cited suggests a value of α close to 2.4. The refringence of the universe may indeed be responsible for this. If we accept this approach, we are led to a lengthening of the age of the universe: it could reach 33 billion years. This would allow it to host in its relatively young phases objects that are already old and structured in relation to the duration scales established for them elsewhere.