Well the pure language in physics lies only in mathmatics. But I don't speak that. So we have to use layman terms. But one of the big indicators is that everything is moving away from each other. Well not everything. We have clusters of galaxies that are locked together by gravity but all the clusters are flying away from each other. Actually the space between them is expanding. Plug in the mathmatics, throw the current process in reverse, and everything comes to a single point about 13.7 billion years ago.
Everything I needed to know about life I learned on Dagobah.