You actually can.....there is one fact that is absolutely proven and undeniable........the fact that the universe is expanding ( 63km per second per MegParsec)......that allows for two other undisputed facts.....1) At any point in the future the Universe will be larger than it is now and ....2) at any point in the past, the universe was smaller than it is now........That second fact allows us to "rewind" the universe which we have done very accurately using the equations of GR, as I have said, light, acts as a time portal, it allows us to look back into time and confirm our findings........as I have said, the Big bang theory is so intricate, that some parts can be disputed , but overall it did happen, because...it had to happen for the universe to be what we see today.........there is a great article in this months Astronomy magazine about how over the last century many of the top minds have tried to debunk the Big bang theory , and none have come close , in fact the name "Big Bang" was given to the theory to try to down play it as stupid........but it has held pat for almost 100 years