Matter/mass exist into the space. Where does space exist? It is said that before big bang there was nothing. Not even space/time. So, now where does space exists?
There is still no way to solve this question for all people.
You also need to keep in mind that time is directly linked to space. So before the universe there was no space and no time. However, if you want describe processes or even states you should also be able to talk about time.
Time is the perception of a sequence of events. So before the big bang no events could happen. To be more detailed time is what physicsists call the increase of entropy. Entropy itself is the randomness of a closed physical system. Each physical systems wants to have maximum entropy or randomness. The big bang at point of time -zero- can be seen as a point with the lowest possible value of entropy, because any matter/energy was located at a single point - at a singularity. There was ne place for particles to get at a random place.

With the expansion of this point space and time expanded into "nothing". If you think about it: how could it expand if there was something?

Next: If there is something outside, we still have a neverending question: What's outside the outside? Is it infinite?
Our physical laws don't work outside - so how should we talk about things nobody could ever describe?

Of course, there are theories that try to explain why or how the big bang happened - the ethereal expansion by Alan H. Guth and Alexander Vilenkin, for example. Universes are "created" like holes in a cheese by massive discharges of energy (like lightnings during a storm), but those are very hypothetical theories without any prove or even evidence. And all those theories need parallel universes / multiverses.

So, for my opinion, I keep at this point: There was nothing. As human beings we cannot deal with nothing. For us there is always something. If we try to think about nothing, we still think on how much time has passed not thinking about anything - for example. Nothing is mathematically described as 0. But also 0 is a number. It's more than nothing. It still consumes place in a formula. So don't think about nothing as "nothing". Think about nothing as everything. Because or universe was created out of nothing, this nothing is also everything for any or every time.