The discovery of the speed of light provided a satisfactory explanation as to why the world becomes dark at night. This is immensely important as the further a star is from an observer, the longer it will take for light to arrive from that point, to the observer themself. If the universe was infinite in its age, all of the light ever created would have had time to reach the Earth which would cause the night sky to be ablaze with starlight, photons emitted from every single possible part of the universe. This clearly is not the case so it is abundantly evident that the universe must have had an origin point. Scientists use the term “observable universe” as it’s entirely possible and likely that matter exists outside of what can be observed predicated upon the theory that as the universe expanded, light from this matter would have to travel increasingly larger distances to reach an observer. Because of constant expansion, this light has not yet had the chance to reach the Earth and perhaps never will, causing it to be outside of an observers field of vision and therefore unable to be proven in its existence using current methods and technologies
