Would recommend reading Chapter 3 "From Eternity to Here" by Sean Carroll (or maybe read the entire book) for a "light weight" introduction. If you don't have the time then TL;DR : universe is expanding means literally that, i.e it is almost as if new space is getting created rather than "cosmological scale objects" hurtling away from each other because they "gained momentum (if one can even call it that)" because of that big bang.