The concept of the expansion of the universe can be a bit counterintuitive, but it does not mean that something is expanding into "nothingness." Instead, it refers to the expansion of the fabric of space itself.
According to the Big Bang theory, the universe originated from a highly dense and hot state about 13.8 billion years ago. Since then, the universe has been undergoing expansion. The expansion of the universe does not involve objects moving away from a central point into empty space. Instead, it is the very fabric of space that is stretching.
To visualize this, you can think of the universe as a rubber sheet with dots representing galaxies or objects. As the rubber sheet stretches, the dots move apart from each other. In this analogy, the dots themselves don't move, but the space between them expands.
It's important to note that the expansion of the universe is not a motion through space, as we typically think of objects moving from one point to another. Instead, it is the stretching of space itself. As a result of this expansion, the distances between galaxies increase over time.
However, it's worth mentioning that the question of what the universe is expanding into is still a topic of debate and not fully understood. Some theories propose the existence of a multiverse or higher-dimensional structures beyond our observable universe, but these ideas are still speculative and require further exploration and evidence.