That there was a finite time in the past in which all matter was squeezed tight and hot enough to undergo primordial nuclear reactions, and that the universe has expanded since, is pretty dang incontrovertible at this point. The nature of that event is still up for debate. Examining the fine details of light of different ages continues to reveal things, like the expansion appearing to initially slow followed by accelerating which is currently best explained by a dark energy model which is also favored for reasons relating to physical models of space I am unprepared to examine.