In multiverse theories, the concept of "empty space" outside of individual universes can vary depending on the specific hypothesis being considered. It's important to note that different multiverse theories exist, each with its own set of assumptions and implications.
In some multiverse frameworks, such as the bubble universe or inflationary multiverse theories, the universes are thought to exist as separate, isolated regions or bubbles within a larger spacetime. These bubbles can have their own distinct set of physical laws and properties. In this case, the space between these bubble universes might be considered "empty" or devoid of matter and energy as we understand them, but it is still part of the overall multiverse structure.
In other multiverse models, such as the many-worlds interpretation of quantum mechanics, the different universes are considered to exist simultaneously within the same overarching space and share a connection through quantum probabilities. In this interpretation, there may not be a notion of "empty space" outside of individual universes because they all occupy the same underlying space.
It's important to recognize that multiverse theories are still largely speculative and under active scientific investigation. There is no definitive experimental evidence that confirms the existence of other universes or the specific characteristics of their connections. Different hypotheses propose various mechanisms for the existence and interactions of multiple universes, and they can differ in their implications for the nature of space outside of individual universes.
As our understanding of physics and cosmology advances, ongoing research and empirical investigations may shed further light on the potential existence and nature of other universes, as well as the structure of the multiverse, including the properties of space between them.