There have been a number of scientific observations which led to the development of the Big Bang Theory. There observations are reported as observable fact without editorial judgement.
1] the spectra of light reaching us from distant stars appears shifted towards the red (lower energy) wavelengths.
2] there is an observable background radiation of 3 Kelvin everywhere we look.
Edwin Hubble – red shifts and the expansion of the universe
The Cosmic Microwave Background
Fr. Georges Lemaitre:
Here’s what the Big Bang Theory says: