Darkness, not brightness, is the real clue that the universe has limits. A sky filled with stars should, by simple geometry, stack their disks along every line of sight until the entire dome glows as bright as an average stellar surface, a puzzle known as Olbers’ paradox in classical astronomy.
The resolution starts with a blunt fact: stars are temporary. Nuclear fusion gives each one a finite lifetime, so there has not been infinite time to pour light into space, and the observable universe itself has a finite radius, which caps how many stars can ever send photons our way, no matter how patient any observer might be.
More striking is what cosmic expansion does to that light. As space stretches, it drags photons with it, increasing their wavelength through cosmological redshift and draining their energy, so radiation that might once have been blisteringly bright now arrives as a thin cosmic microwave background, barely above absolute darkness, leaving most of the sky nearly black.
