The concept of "before" the Big Bang is still a topic of active scientific investigation and remains a subject of debate and speculation. The current understanding based on the Big Bang theory suggests that the universe as we know it began with the Big Bang, including both space and time. However, our understanding of what happened at the instant of the Big Bang and whether there was something "before" it is limited.
When scientists say that space and time didn't exist before the Big Bang, it means that our current understanding of space and time breaks down as we approach the singularity, the initial state of the universe. According to the Big Bang theory, the universe was in an extremely hot, dense, and compact state at the singularity, but the laws of physics as we understand them cannot accurately describe what occurred at that point.
Regarding the question of whether there was "nothing" before the Big Bang, it's difficult to make definitive statements due to the limitations of our current scientific understanding. The concept of "nothing" is complex and can be difficult to define. Some theories propose the existence of a pre-Big Bang era or a multiverse scenario, but these ideas are still highly speculative and the subject of ongoing research.
It's important to note that our current understanding of the early universe and the Big Bang is based on mathematical models and scientific observations. As our understanding of physics and cosmology evolves, our ideas about the nature of the universe before the Big Bang may also change.