Ever since I first heard about the Big Bang as a kid, I’ve never really bought it. The concept of the entire universe being literally a dimensionless point - I just don’t think so. If that’s what the math leads to, doubt the math or the observations the math is based on. Same with dark matter and dark energy - I mean come on, if a theoretical model of the universe says it has to be 20x more dense than we can measure, you rethink the model - don’t decide 95% of everything must be “dark”. Dark is for the 3rd movie in a superhero franchise when the 2nd one doesn’t make enough money, it’s not a way to define the universe.
/end rant
Well, there was a big bang, but the Big Bang Theory doesn’t talk about the dimensions of empty space, just matter. There are several alternate theories that, for example, say the Big Bang is a reoccurring phenomenon, like all the contents collapose and it bangs again or bangs just happen rarely in empty space, but outside of distances we can observe.
About dark matter/energy: That’s really just a fancy label for the math being off a bit. It might turn out as a formula/constant not being perfect or it’s just something that’s really hard to spot, like a UFO can just turn out to be some swamp gas. Something’s there, but we don’t know until we find out more.