Since Tony pointed out the my comment was both oversimplified and mistaken, I kinda wanted to hear his in-depth explanation.

Your original statement...

"Before the big bang, there was nothing. And then that nothingness exploded."

... assumes that scientists have some sort of idea about what the universe was like before the big bang. They don't, not really. At least not in any concrete form, it's all very theoretical. You also say that "the nothingness exploded", but the current theory doesn't assume that matter exploded from nothingness. It's just that we don't know where the matter came from and we're making a lot of guesses about how it might have happened. They're just guesses at this point.

The original idea of the big bang theory comes from the observation that the universe is expanding. The idea is that the center of this expansion is the origin of the known universe, and that the expansion is the result of an explosion. It's not much more than that basic concept. Everything we're observing now seems to be consistent with that concept, but you have to admit, the vantage point from which we're viewing the universe right now is quite limited, so it all has to be taken with a grain of salt. You can't say, "Scientists believe that there was nothingness and then the nothingness exploded", because they really don't believe that. All they see is evidence that the universe as we know it appears to be the result of an expanding shockwave from a billions-of-years-old explosion. Anything beyond that, and they're still working on it.

This is different from other scientific discipines like biology or chemistry where we can directly observe the processes involved, and can get our hands on all the details and evidence we need. Some theories are just wild theories, others are a lot more concrete and tangible. We're getting a reasonably good handle on the workings of this planet, but the farther out you go from earth, the less we know and the more theoretical stuff becomes.