The multiverse is a hypothesis about the structure of our universe outside the region accessible to our observation. This implies that the speed of light is limited, and the universe is expanding in all directions. At the same time, people can only see a certain part of the space (Horgan). And it is far from the fact that the world outside it is arranged in the same way as in the vicinity of the Earth. Hypothetically, outside the observable sphere, for example, there may be a completely different ratio of ordinary and dark matter.
The problem here is that nothing can travel faster than light through space. So, at any given moment, the speed of light limits how far we can see. This limit is called the cosmological horizon (Horgan). Any carrier other than light will be slower or, in the case of gravity itself, as fast as light. Therefore, if an object is so far away that the light from it has not yet managed to reach it, then people have no right to claim that this object is generally there somewhere.
However, it seems reasonable to note that although nothing can move faster than light in space, space itself does not know such limits. It can expand, which it does in the multiverse, faster than light, and therefore there are regions from which light will never reach us. In the multiverse, all other universes are located in such areas and therefore are not causally connected with us (Horgan). Therefore, the opponents of the multiverse say people will never be able to subject it to any measurements, which means that it does not belong to the realm of science.
Work Cited
Horgan, John. “Multiverse Theories Are Bad for Science.” Scientific American, 2019, Web.